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North Bondi Apartment: Mould in the Aircon Return Air Cavity
By Michael Nguyen | Indoor Environmental Professional (IEP) | IICRC WRT, AMRT, ASD (69486970) | Mould Testing Technician (NAT10913005) | Licensed Builder NSW 392154C (Supervisor, KBL+RENO)
Quick answer: Mould was growing through the insulation inside the air conditioner return air cavity of a North Bondi apartment. We contained the area and put it under negative air, removed the mould-infested insulation and HEPA vacuumed the cavity. The moisture came from high indoor humidity, and the one-day job cost $1,500 + GST.
The return air is where the air conditioner draws air back from the room. If the cavity behind the grille gets damp, mould growing there can be drawn straight into the system and blown around the apartment every time the aircon runs.
The Job
On 13 November 2026 we attended an apartment unit in North Bondi, in Sydney's Eastern Suburbs. Inside the aircon return air cavity, the insulation was heavily affected by mould. In an apartment, the strata manager may also need to be kept informed if common property or other units could be affected. The scope:
Contain the work area and put it under negative air
Remove the mould-infested insulation from the return air cavity
HEPA vacuum the cavity
What Caused It
The moisture came from damp air inside the apartment: high indoor humidity. When humid air reaches the cooler surfaces around an air conditioner, moisture can condense, and the insulation in the return air cavity stays damp long enough for mould to grow.
To help stop it coming back, keep indoor humidity down: open up and ventilate when you can, use bathroom and kitchen exhaust fans, run the aircon on dry mode in humid weather, or use a dehumidifier.
Why Containment and Negative Air Come First
Disturbing mouldy insulation releases a lot of spores and fine debris. Before anything is removed, we seal the work area with plastic sheeting and a zip door, then run a HEPA air scrubber to keep the containment under negative air. Air is drawn into the work area, not out of it, so spores stay where the work is happening and away from the rest of the apartment.
What We Did
Containment. Plastic sheeting and a zip door around the return air.
Negative air. A HEPA air scrubber kept the containment under negative pressure.
Source removal. The mould-infested insulation was removed from the cavity.
HEPA vacuuming. The cavity was HEPA vacuumed to remove settled spores and debris.
Standards We Follow
Our mould work follows ANSI/IICRC S520, including containment, negative air, source removal and HEPA cleaning. The source of the moisture has to be dealt with, or the mould will come back. Here the source was high indoor humidity, so keeping the apartment's humidity down is what stops it returning.
What It Costs
This was a one-day job and cost $1,500 + GST. There was no clearance test (PRV) on this job. Our mould inspections and quotes are free, and professional moisture and mould reports start from $500 + GST. See our mould removal price guide for more.
For more on our work in the area, see our Eastern Suburbs mould removal page, or read about another aircon job this week: an aircon drip tray leak in a Maroubra apartment.
About the Author
Michael Nguyen is the director of AAA Mould Removal and leads our water damage and mould remediation work across Sydney. His qualifications include:
Indoor Environmental Professional (IEP) – Testing Water-Damaged Buildings, Australian College of Environmental Studies (RTO 21740), 2026. Includes nationally recognised units NAT11429003 Conduct a comprehensive mould and moisture assessment and NAT11429007 Conduct post remediation verification mould testing (11429NAT Advanced Diploma of Building Biology)
IICRC Water Damage Restoration Technician (WRT)
IICRC Applied Microbial Remediation Technician (AMRT)
IICRC Applied Structural Drying (ASD), IICRC no. 69486970
Mould Testing Technician (NAT10913005)
Licensed Builder NSW 392154C (Supervisor, Kitchen, Bathroom & Laundry and Renovation)
Carpentry Licence
AAA Mould Removal is an IICRC Certified Firm. Michael is also completing a double degree in Medical Science and Environmental Science at Macquarie University.
Worried About Mould or Water Damage?
Call AAA Mould Removal on 1300 739 783 or contact us online. We will find the moisture, remove the mould properly and dry your home to IICRC standards.
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