Why Your Air Conditioning Installation Probably Blows (And Not in the Right Way)

Most of the air conditioning complaints I hear from homeowners come back to one thing: the system wasn't installed properly. The usual symptoms are:

• Inadequate cooling or heating: Systems failing to perform at the expected capacity.
• Excessive noise: Poorly placed units or improperly installed ductwork causing disturbances.
• High energy bills: Systems running inefficiently due to improper installation or non-compliance with standards.
• Water leaks or condensation issues: Often a result of incorrect piping or lack of proper drainage planning.

Australian Standards in Air Conditioning Installation

The key standard for ducted air conditioning in Australia is AS 4254.1 (Ductwork for air-handling systems in buildings, Part 1: Flexible duct). It covers duct sizing, insulation, sealing, labelling and hanger supports. Get it wrong and you get air leaks, poor airflow and bigger power bills.

A note on editions: this post was written against AS 4254.1-2012. The standard was revised in 2021 (AS 4254.1:2021), and the revision changed the hanger support and duct taping requirements. The clause numbers and figures quoted below come from the 2012 edition, so ask your installer which edition they're working to and check the current figures.

Below I cover the two installation stages, the common mistakes, what to check at rough-in and fit-off, and what to do when it goes wrong.

Understanding Air Conditioning Installation Basics

Stages of Installation: Rough-In and Fit-Off

Rough-In Stage: The rough-in stage generally occurs after your fascia gutter and roof is installed and other services like plumbing and electrical are roughed in. The AC rough in involves laying out the infrastructure required to support your air conditioning system, such as ductwork, piping, and electrical wiring.

Key points during the rough-in stage:

• Ductwork layout: Properly sized and placed ducts are essential for efficient airflow. If ducts are too small or poorly placed, the system will struggle to maintain temperature and airflow, working harder than it needs to (using more electricity to do so).
• Refrigerant piping: Accurate piping sizing ensures the right amount of refrigerant flows between the indoor and outdoor units. Errors here can cause inefficient cooling or heating and increase energy costs.
• Electrical wiring: The right wiring and circuit sizing is necessary to power the system without overloading circuits or creating safety hazards.

Fit-Off Stage: Fit-off happens near the end of the build, after painting and once the other trades have fitted off. The installer fits the ceiling or wall diffusers (the vents), connects the ducts to them, mounts the outdoor unit and connects it to the roughed-in pipework and power, then wires up the controller and commissions (sets up and tests) the system.

Key points during the fit-off stage:

• Sealing ductwork: Proper sealing ensures there are no air leaks, which could compromise system efficiency and increase energy consumption.
• Calibrating the system: Ensuring the system is calibrated to the right temperature and pressure settings maximises performance.
• Drainage system installation: Correct installation of the drainage system (condensate line) prevents issues like water leaks and mould growth.

C10 - Rough In Stage Inspection Checklist

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Last updated 28 Oct 2026 · Revision 3 · Free updates for life

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When to use: Rough-in stage: after services are installed, before cladding and linings go on

An 84-point rough-in checklist. Check plumbing, electrical, gas, solar, air conditioning and data after every trade has finished, before the insulation and linings hide them.

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Types of Air Conditioning Systems: Ducted vs. Split Systems

Ducted Systems: A ducted system pushes cooled or heated air through a network of insulated ducts to vents in each room. Ducted suits larger homes where you want to control several rooms, but it generally costs more to buy and run, and needs a bigger power supply.

Split Systems: A split system has one indoor unit and one outdoor unit, and conditions a single room or space. Splits are often more energy efficient than ducted because you only turn on the rooms you're using, rather than running a central unit and closing off zones. In my opinion, splits are the way to go for energy efficiency and cost.

The Role of Ductwork: Why Flexible Duct Compliance is a must

AS 4254.1 sets the rules for flexible duct installation: how ducts are sealed, insulated and supported.

Key elements of ductwork installation include:

• Sizing: Incorrectly sized ducts can restrict airflow, causing the system to work harder and increasing energy costs. AS 4254.1 provides guidelines on how to size ducts correctly to ensure optimal performance.
• Duct run lengths: Duct runs should not exceed 6 metres for any one duct diameter, so airflow isn't choked over long distances.
• Houses and other Class 1 to 3 and Class 10 buildings (as classified under the National Construction Code): use no more than two lengths of flexible duct, each up to 6 metres, between air terminal devices (the vents).
• Upsizing: Going one duct diameter larger than specified is sometimes recommended to reduce pressure loss.
• Sealing: Ducts must be properly sealed using adhesive tapes or drawband systems to prevent air leaks.
• Peeling back insulation: When connecting flexible duct to collars, vents or joiners, the outer jacket and insulation must be folded back to expose at least 100 mm of the inner duct core, so the core can be sealed and fastened properly.
• Taping: Duct tape must go on with at least three overlapping, tensioned, crease-free wraps, each overlapping the last by 40% to 60% of the tape width. Any leak drags down efficiency and leads to uneven cooling or heating.
• Insulation: Proper insulation of ducts helps maintain the desired temperature as air travels through the system. The standard outlines the R-value (thermal resistance) required for different climate zones across Australia.

Labelling Requirements for Flexible Ducts

AS 4254.1 also requires flexible ducts to be labelled, so you can tell what was installed and whether it meets the insulation requirements. According to the standard:

• Labelling must be repeated along the length of the duct every 1000 mm.
• The label must include the following information:
• the name of the manufacturer
• compliance with AS 4254.1
• the R-value of the insulation (its thermal resistance)
• The labelling must stay legible for the life of the duct, with characters at least 10 mm high.

Common Air Conditioning Installation Mistakes

Even a good unit performs badly if it's installed badly. These are the mistakes I see most.

1. Poor Placement of Outdoor Units

The placement of the outdoor unit is essential to the overall performance of the system. A poorly positioned outdoor unit can cause airflow blockages, noise disturbances, increase maintenance and reduced efficiency.

Key issues with improper placement include:

• Blocked airflow: If the unit is placed too close to walls, fences, or other objects, it won’t have enough space for proper ventilation. This leads to poor cooling performance and higher energy consumption.
• Excessive noise: Units placed near bedrooms or living spaces can produce disruptive noise, particularly if vibrations from the compressor aren't dampened. Noise barriers or proper placement away from sensitive areas can mitigate this problem.
• Increased wear and tear: Improper placement can expose the unit to harsh environmental elements such as direct sunlight, dust, and debris, which can lead to faster deterioration.

2. Incorrect Duct Sizing and Sealing

Proper duct sizing and sealing are essential to maintaining airflow and energy efficiency. Poor ductwork is one of the most common causes of inefficient air conditioning systems, leading to uneven temperature distribution and higher energy bills.

Common issues related to ductwork include:

• Incorrectly sized ducts: Ducts that are too small or too large reduce airflow and strain the system. Under AS 4254.1, duct sizing must be calculated to suit the building's load.
• Air leaks: Sealing ducts is critical to prevent air from escaping, which can account for significant energy loss. AS 4254.1 outlines proper sealing methods using approved tapes and drawband systems. Even small gaps in the ductwork can cause a dramatic drop in system efficiency.
• Poor insulation: If ducts are not insulated properly, the cooled or heated air can lose its temperature before reaching its destination, making the system work harder. Section 2.6 of AS 4254.1 provides detailed insulation requirements to prevent heat loss or gain through ducts.

This technical paper on new construction looks at the envelope line and ceiling space, and how duct sizing affects whether the system can actually be built as designed:

3. Electrical Wiring Mistakes

Air conditioning systems require complex electrical wiring to function safely and efficiently. If the electrical system is not installed according to the manufacturer’s specifications or AS/NZS 3000, it can result in safety hazards or equipment failure.

Common electrical mistakes include:

• Insufficient breaker sizing: If the system’s electrical load is greater than the breaker’s capacity, it can result in frequent tripping or, worse, a fire hazard.
• Poor earthing: The system must be properly earthed to avoid electrical hazards. The Daikin Installation Manual stresses that wiring must be done by a licensed electrician.

Check out the VBA Technical solution below that consolidates the standards and industry best practices into a handy document:

What to Look for at the Rough-In Stage

Rough-in is the most important stage to check (true for all trades, really). It's far cheaper to ask questions and fix things before the plasterboard goes up and access disappears.

1. Ductwork and piping installed to AS 4254.1

Check that all ductwork and piping complies with AS 4254.1 for sizing, sealing, insulation and support.

What to check:

• Duct size and layout: Check that the ductwork is properly sized to accommodate the system's load requirements.
• Refrigerant piping installation: The refrigerant lines must be correctly positioned and insulated to avoid temperature loss. Pay attention to the connections between the indoor and outdoor units, as improperly installed refrigerant lines can lead to leaks and cooling inefficiency.
• Compliance with AS 4254.1: This includes ensuring that ductwork is supported correctly, with hangers spaced appropriately as per section 2.5.3 of the standard. Improper support can lead to sagging ducts, air leaks, and pressure loss, all of which reduce the efficiency of the system.

2. Flexible duct hangers, support and load distribution (AS 4254.1)

Flexible ducts are particularly prone to sagging or kinking, which can severely restrict airflow.

Key points to ensure:

Is your duct properly hung?

• Hanger supports: The ductwork should be supported at intervals recommended by the standard to prevent sagging. Section 2.5.3 of AS 4254.1 outlines the correct placement of hangers to ensure proper load distribution and to prevent air pressure drops.
• Independent support for vents: Air terminal devices (vents) and branch take-offs must be supported independently of the flexible duct.
• Maximum sag: No more than 40 mm of sag per metre between supports.
• Support spacing: Flexible duct must be supported at intervals of no more than 1.5 metres, and the supports must keep the duct at its full internal diameter, with no kinks or flattening. Simply laying flexible duct over trusses without hangers is not adequate support under the standard, even with trusses at 600 mm centres.

Installers read this one differently. Insulated duct laid over trusses at 600 mm centres is arguably better supported than duct on hangers at 1.5 m spacing. But to be legally compliant, the installer would need a performance solution showing it's acceptable. I don't like lawyers, so I just do what the standard says.

• Load distribution: Ducts must be evenly supported to avoid stress points that could lead to air leaks or duct collapse. This is especially important for long duct runs.

3. Duct placement: avoiding pressure loss and airflow restrictions

Ducts that are kinked, badly placed or too long cause:

• Restricted airflow: Kinks or bends in the ducts can drastically reduce airflow, leading to uneven cooling or heating in different parts of the home.
• Pressure loss: If ducts are too long or not correctly placed, the system will have to work harder to push air through, increasing energy consumption and wear on the equipment.

4. Insulating and sealing ducts with tape and draw bands

At rough-in, check every duct is insulated and sealed with approved tapes or a draw band system (a clamp or tie that pulls the duct tight onto the fitting).

Important things to check:

• Sealing with approved tapes: Tapes used for sealing must comply with Section 2.2.4 of AS 4254.1 to ensure long-lasting and effective seals. Leaky ducts can cause up to 20% energy loss.

5. Outdoor unit location

Lock in the outdoor unit location at rough-in, using the clearances in the manufacturer's installation manual (the Daikin manual is a good example). Check the placement points covered above: room for airflow, shelter from harsh sun and weather, and noise near bedrooms, yours and the neighbours'.

Key Considerations at the Fit-Off Stage

Fit-off is where the system is connected, tested and commissioned. Here's what to watch.

1. Air and vapour sealing (AS 4254.1, Section 2.2.4)

Check that every duct connection is sealed with approved tapes, mastics and clamps, as required by Section 2.2.4 of AS 4254.1.

Key points to monitor:

• Air sealing: Ensure all duct connections are sealed with approved tapes and clamps to prevent air leakage, which could reduce system efficiency and lead to uneven cooling or heating.
• Vapour sealing: Proper vapour sealing prevents condensation inside the ductwork, which can lead to mould growth, water damage, and reduced system lifespan.

2. Verifying the System’s Calibration and Performance Testing

Once the system is physically connected, it needs to be calibrated to ensure it operates at peak efficiency. This involves adjusting the system’s airflow, refrigerant levels, and thermostat settings.

Important steps include:

• Airflow calibration: The airflow needs to be adjusted to ensure each room receives the appropriate amount of cooled or heated air. Inadequate airflow can result in uneven temperatures, reduced comfort, and increased energy consumption.
• Refrigerant levels: The refrigerant lines must be charged to the correct pressure levels as per the manufacturer’s guidelines. Improper refrigerant levels can lead to inefficient cooling or heating, compressor damage, and higher energy usage.
• System performance testing: After calibration, the system should be tested under full operational conditions. This includes running the system for a designated period to ensure it meets the required performance standards.

You really know your life is rocking when you've read this (the Daikin Installation Manual).

It has useful information, especially on clearances and locations for outdoor units. It's actually worth a read. You're welcome 😸

3. Duct leakage testing (AS 4254.1, Section 2.2.7)

Under Section 2.2.7 of AS 4254.1, ductwork should be tested to confirm it's airtight. Leaks waste energy, push up running costs and leave some rooms hotter or colder than others.

Key steps:

• Pressure testing: The installer should conduct a pressure test to check for any leaks in the duct system. This ensures that no air is escaping, and that the system is delivering maximum efficiency.
• Visual inspection: After pressure testing, a visual inspection should be performed to check for any gaps, tears, or incomplete seals that might have been missed during the rough-in stage.

4. Drainage System Installation

Air conditioners produce condensate (water pulled out of the air), and it has to be drained away properly or you get leaks, stains and mould.

Key considerations:

• Proper drainage slope: Ensure that the drainage pipes are installed with the correct slope to allow water to flow freely away from the system.
• Condensate pump: In cases where gravity drainage isn’t possible, a condensate pump may be required to move water away from the system. Ensure that the pump is functioning properly and tested during the fit-off stage.
• Drainage outlets: Verify that the drainage outlets are positioned in a way that prevents water from pooling near the building or entering the living areas.
• The condensate line should discharge into the stormwater system with at least a 50 mm downturn into the downpipe. Condensate carries copper from the coil, which stains metal roofing. More on this in the presentation below:

5. Electrical and Safety Testing

Before the system can be considered fully operational, the electrical wiring must be tested for safety and compliance with Australian standards (AS/NZS 3000). This ensures the system runs smoothly without overloading circuits or posing a fire hazard.

What to check:

• Breaker capacity: Ensure that the system is connected to a circuit breaker of appropriate capacity. Using the wrong breaker size can lead to frequent tripping or, in extreme cases, a fire.
• Earthing: The system must be correctly earthed to prevent electrical hazards.
• Safety tests: All electrical connections should be inspected, and tests such as earth continuity and insulation resistance should be performed to ensure the system is safe to operate.

6. Receiving a flexible duct compliance report summary

Have you ever seen or heard of a flexible duct compliance report summary? Yeah, my people have never given me one of these!

This report, completed by the installer, certifies that the ductwork complies with AS 4254.1, including the testing standards specified in the regulation.

With more substandard imported materials on the market, this report is your evidence that the duct in your home meets the Australian Standard. That matters if there's ever an insurance claim or a dispute. Here's what you're looking for:

Your Air Conditioning Installation Checklist

Here's the short version to use on site before you pay. The C10 Rough In checklist above covers the other trades at the same stage.

• Duct sizing: Verify that the ducts have been sized correctly based on the system’s cooling or heating load.
• Sealing: Ensure all duct joints and connections are properly sealed using approved methods such as adhesive tapes and draw band systems, as outlined in Section 2.2.4 of AS 4254.1.
• Insulation: Check that all ductwork is properly insulated to prevent temperature loss as air moves through the system. The insulation should meet the R-value requirements outlined in the standard.
• Support and hangers: Confirm that the ductwork is supported by hangers and brackets at appropriate intervals (as per Section 2.5.3 of AS 4254.1) to avoid sagging and air leakage.
• Refrigerant lines: Connections tightened to the manufacturer's specification and pressure tested for leaks.
• Airflow balanced: Airflow measured at each vent and dampers adjusted so every room gets its share.
• Thermostat and controller: Calibrated and reading the actual room temperature.
• Condensate drain: Falls freely to a proper outlet, with any condensate pump tested.
• Electrical: Breaker sized to the system's load, system properly earthed, and tested to AS/NZS 3000.
• Paperwork: Flexible duct compliance report summary, commissioning results, and both the unit and installation warranties.
• Independent check: For peace of mind, have a third-party inspector review the installation before you sign off.

Dealing with Defects: What to Do When Things Go Wrong

Even with good planning, things go wrong. Here's how to handle it.

1. Steps to Address Defects

When you encounter a defect, it’s important to address it quickly to prevent further issues. Here’s a step-by-step guide on how to handle common air conditioning defects:

• Document the issue: Take notes and, if possible, photos of the defect (e.g., water leaks, unusual noise, temperature inconsistencies). This documentation will help when communicating with the installer or manufacturer.
• Contact your installer: Notify the installer as soon as you notice a problem. Most air conditioning systems come with warranties that cover defects in workmanship for a specified period. A professional should inspect the system to determine the root cause of the issue.
• Schedule an inspection: If the issue isn’t immediately resolved, request a professional inspection of the entire system, including ductwork, refrigerant lines, and electrical connections. This is particularly important for defects related to airflow, condensation, or noise.
• Ask for a compliance check: Ask the installer to check the work against AS 4254.1 and the manufacturer's installation manual, so you know whether the defect comes from a non-compliant installation. If it does, you may be entitled to repairs or compensation.

2. Understanding Your Rights Under Australian Consumer Law

In Australia, home owners have certain rights when it comes to defective products and services. If you’ve had a substandard air conditioning installation, you’re entitled to remedies under Australian consumer law.

Key rights include:

• Repairs, replacement, or refund: If the defect is substantial or cannot be easily fixed, you may be entitled to a repair, replacement, or refund of the air conditioning system. This applies to both the unit itself and the installation services.
• Warranty enforcement: Most air conditioning units and installations come with a warranty period. Ensure that the installer honours the terms of the warranty by providing free repairs for defects that arise within this period.
• Lodging a formal complaint: If your installer refuses to address the defect, you can lodge a formal complaint with consumer protection agencies or seek legal advice to recover the costs of repairs or replacement.
• Building a new home in Queensland? If the air conditioning was installed under your building contract, take it up with your builder, not the installer. Your contract's defects period (12 months from practical completion under the QBCC New Home Construction Contract) and the QBCC complaints process apply.

3. Ensuring Warranties Cover Both Unit and Installation Issues

When it comes to warranties, it's important to distinguish between the unit warranty and the workmanship warranty for the installation itself. Here's what to watch out for:

• Unit warranty: Most manufacturers, like Daikin, offer a warranty for the unit covering any defects in the product.

Be sure to register your unit after installation to activate this warranty.

• Installation warranty: Your installer should provide a separate warranty for the workmanship involved in installing the system. This warranty ensures that any issues arising from incorrect installation are fixed at no extra cost to you.
• Extended warranties: Some companies offer extended warranties for both the unit and installation. Consider opting for this if your home is in a high-wear environment, such as coastal areas prone to salt damage.

4. Negotiating Payment Withholding or Adjustments for Non-Compliant Installations

If the installation has defects or doesn't comply, raise it before you pay. If you engaged the installer directly:

• Document the issues: Take notes and photos of any problems you find during the inspection. This documentation will be useful when discussing repairs or payment adjustments with the installer.
• Request corrections: Notify the installer of any defects and request that they fix the issues before you make the final payment. Be clear about what needs to be corrected and reference AS 4254.1-2012 or the Manufacturers Installation Manual if necessary.
• Withhold payment: If the installer refuses to address the issues, you may need to withhold part of the payment until the defects are fixed. In some cases, you can negotiate a reduced payment if the installer agrees to correct the defects later.
• Know which law applies: If you're a homeowner building your own home, your rights come from your building contract and the QBCC Act. The Building Industry Fairness (Security of Payment) Act 2017 (BIF Act) payment claim rules generally don't apply to domestic building contracts with a resident owner. If you're a builder paying a subcontractor, the BIF Act timeframes do apply: answer a payment claim with a payment schedule on time, or you can be liable for the full amount.

For builders and trades, here's a handy overview of the BIF Act and what to do when:

and below is what a payment schedule looks like:

And if you're the one sending invoices, like we are, this is what a section 99 notice looks like. It's part of the escalation process towards adjudication or default judgement:

• Seek third-party verification: If disputes arise, you may want to hire a third-party inspector to verify the issues and provide an unbiased assessment. This can be helpful in resolving disagreements and ensuring that the installation meets all required standards.

Ensuring Long-Term Efficiency and Maintenance

Even a well-installed system needs regular maintenance to stay efficient and last.

1. Regular Maintenance Tips

Key maintenance tasks include:

• Cleaning filters: Air filters should be cleaned or replaced regularly to ensure optimal airflow. Dirty filters restrict airflow, reducing the system’s efficiency and increasing energy consumption. Depending on the system, filters should be checked every month or two during heavy usage periods.
• Inspecting ductwork for leaks: Duct seals degrade over time and let conditioned air escape. Have ducts checked periodically and resealed if needed, especially where they run through the roof space or under the floor.
• Checking refrigerant levels: Low refrigerant levels can indicate a leak and will reduce the system’s ability to cool or heat effectively. If you notice a decline in performance, have a professional check the refrigerant charge.
• Cleaning the outdoor unit: The outdoor condenser unit can accumulate dirt, leaves, and debris, which can restrict airflow and reduce efficiency. Make sure to clean the unit regularly and ensure that nothing obstructs its airflow. (this is a big issue with many home owners - piling things in front of the outdoor unit).
• Checking the condensate drain: Ensure that the drain line is clear of blockages. A clogged drain can cause water to back up into the system, leading to water damage or mould growth. Condensate pumps, if installed, should also be inspected regularly to ensure proper functioning.

2. How Often Should Professional Maintenance Be Performed?

While home owners can perform some basic maintenance tasks, more in-depth inspections should be done by a professional.

Professional maintenance should include:

• Annual or biannual inspections: A licensed technician should inspect your system at least once a year, ideally before the cooling or heating season begins. In regions with extreme temperatures, biannual checkups are recommended.
• Comprehensive checks: During a maintenance visit, the technician will inspect refrigerant levels, calibrate the thermostat, check for leaks, clean coils, and perform safety checks on electrical components.
• Ductwork inspection: A professional can perform a more thorough inspection of the ductwork to identify leaks or obstructions that may not be visible to the home owner.

3. Preventative Measures to Extend System Life

There are several preventative measures home owners can take to help extend the lifespan of their air conditioning system:

• Install a programmable thermostat: A programmable or smart thermostat allows you to optimise the system’s performance by setting temperature schedules that match your daily routine. This reduces unnecessary wear and tear by avoiding overuse during periods when cooling or heating isn’t needed.
• Ensure proper insulation: Well-insulated homes reduce the load on air conditioning systems by maintaining indoor temperatures for longer periods. If your home is not properly insulated, the system will have to work harder to maintain desired temperatures, which can lead to reduced efficiency and premature failure.
• Seal windows and doors: Ensuring that windows and doors are properly sealed reduces air leakage, making the system’s job easier and more efficient.
• Use ceiling fans: Ceiling fans can help distribute cool or warm air more evenly throughout the home, reducing the system’s workload and prolonging its life.

Test yourself

Five quick questions on this topic. Click a question to see the answer.

Further Reading

The Unspoken Reality: Why Contractors Don’t Follow StandardsReady to hire a contractor? Think again. Many contractors use methods that don’t align with regulations or Australian standards. This article reveals why you might hear, “We don’t do it that way here,” and how such shortcuts can risk your home’s quality and safety. Learn to protect your investment.Understanding Nominative and Informative Building StandardsDo you truly understand nominative and informative standards? Present in many sectors, these standards play a significant role in construction. Understanding these distinctions can affect numerous decisions, from choosing materials to planning methods.End of Free Australian Standards: Impact AnalysisThis article discusses the end of free access to Australian Standards in public libraries since 2018, its effects on small businesses, safety compliance, and the balance between public and corporate interests. Options for future access and public mobilization strategies are also explored.Why Australian Standards Are Mandatory for TradesmenIf you’re a tradesperson in Australia, the Australian Standards are essential—it’s the law. Ensure safety and quality in every project. Discover why these standards matter.A Crash Course in Australian Standards for New HomeownersCongrats on your new home! Understand Australian Standards for safety and compliance - they’re the law. Get a crash course to ensure your investment’s security.How ITPs & Oversight Improve Construction QualityInspection Test Plans (ITPs) are the only way to start ensuring construction quality, but without proper regulatory oversight, their effectiveness is limited. In this article, we explore why mandatory ITPs and oversight are the key to improving the quality of residential construction in Australia.Understanding ITPs, ITRs, ITCs in Construction InspectionsLearn the distinctions and roles of Inspection Test Plans (ITPs), Inspection Test Records (ITRs), and Inspection Test Checklists (ITCs) in construction. Read how ITPs provide a systematic plan for checks and tests, while ITRs and ITCs document the outcomes of these inspections.

FAQ

Can flexible duct just be laid over trusses without hangers?

Not under the standard. Laying duct over trusses at 600 mm centres may look like plenty of support, but AS 4254.1 requires hanger supports. Practically you can argue it either way, but compliance is what counts if there's ever a dispute.

What should I check before paying for my air conditioning installation?

During the pre-payment inspection, check the ductwork for proper sealing, insulation, and support according to AS 4254.1. Ensure that refrigerant lines are free of leaks, airflow is properly balanced, and the system performs efficiently without excessive noise. Additionally, confirm that electrical connections comply with AS/NZS 3000.

What if my air conditioning installation doesn't meet the standard?

Ask your builder (or the installer, if you engaged them directly) in writing to fix it. If they won't, escalate: to the QBCC for work under your building contract, or to the Office of Fair Trading for a separate installation contract. Legal advice is the last resort, always!