Mould in Your Aircon: How to Spot It, Remove It, and Prevent It

Why Mould in Your Aircon Is a Bigger Problem Than You Think

You switch on your aircon after a few weeks off, and within minutes there's that smell. Musty, earthy, a bit like a damp towel left in a bag too long. If that sounds familiar, there's a good chance you've got mould in your aircon. It's one of the most common problems Australian homeowners face, and given our humid summers along the coast and in tropical regions, it's almost inevitable if the unit isn't maintained properly.

The smell is unpleasant enough on its own, but mould in an aircon is more than just an odour issue. It can trigger allergies, worsen asthma and circulate spores through every room the system serves. On top of the health risks, mould growth on the evaporator coil and filter forces the unit to work harder, which pushes up your power bill and shortens the life of the system.

This guide covers the three things you need to know: how to spot mould in your unit, how to remove it safely and how to stop it coming back for good.

How to Tell If Your Aircon Has Mould

Mould in an aircon unit doesn't always announce itself with visible growth. Often the first sign is a smell or a change in how the system performs. Catching it early makes the clean-up far simpler and keeps the health risks to a minimum, so it's worth knowing what to look for.

The most common warning signs include:

• A musty or earthy smell when the unit first starts running, or throughout operation
• Visible dark spots, black, green or grey, on the vents, louvres or air filter
• Household members experiencing increased allergy symptoms, sneezing, itchy eyes or respiratory irritation that eases when the aircon is off
• A noticeable drop in cooling or heating performance despite the unit running normally

Any one of these signs is worth investigating. Two or more together is a strong signal that mould has taken hold somewhere inside the system.

Where Mould Hides Inside an Aircon Unit

Mould needs moisture, warmth and something organic to feed on. An aircon unit provides all three in abundance, which is why it's such a common problem. There are four main spots where mould tends to establish itself.

The air filter is the most accessible and the first place to check. Dust and debris collect on the filter surface, and in humid conditions that layer of grime becomes a perfect growing medium. Most filters can be removed and inspected in under a minute.

The evaporator coil sits behind the filter and is constantly damp from the condensation it produces during normal operation. It's dark, it's cool and it stays wet, which makes it one of the most mould-prone components in any split system or ducted unit.

The drain pan collects the condensation that drips off the evaporator coil. If the drain line becomes partially blocked, water sits in the pan rather than draining away, and standing water is a mould magnet. A blocked drain pan is also a common reason for an aircon leaking water inside your home, so if you've noticed drips or water stains near the indoor unit, check the pan first.

The internal ductwork is the hardest to reach and the most serious risk in ducted systems. Because ducts run through walls and ceilings, mould growing inside them can circulate spores through every room in the house without you ever seeing it. If you have a ducted system and suspect mould, a visual inspection of the vents alone won't be enough. A professional inspection with a camera or UV light is the only reliable way to assess the ductwork properly.

Split systems are generally easier to inspect and clean yourself, but ducted systems demand more attention precisely because the problem can spread so far before it's detected.

How to Remove Mould from Your Aircon Safely

Removing mould from an aircon is a straightforward DIY job for most split systems, provided the mould is limited to the filter, vents and accessible surfaces. Before you start, gather your supplies: rubber gloves, a face mask (P2 rated if you have one), safety glasses, a spray bottle, white vinegar, mild detergent and a few clean cloths. The whole process takes about an hour.

1. Turn off and unplug the unit. Switch the aircon off at the wall and, where possible, unplug it or isolate it at the circuit breaker. Never clean any part of the unit while it has power running to it.
2. Put on your protective gear before touching anything. Gloves, a face mask and eye protection should all be on before you open the front panel. Mould spores become airborne the moment you disturb them, and you don't want to breathe them in or get them in your eyes.
3. Remove and soak the air filter. Take the filter out carefully and place it in a bucket or sink with a solution of equal parts white vinegar and warm water, or a small amount of mild detergent. Let it soak for 15 to 20 minutes, then rinse thoroughly under running water. Set it aside to air-dry completely before reinserting. A damp filter put straight back in will make the problem worse.
4. Wipe down accessible surfaces. Using a cloth dampened with a diluted white vinegar solution (roughly one part vinegar to one part water), wipe the vents, louvres and the inside of the front panel. Work methodically and use a fresh section of cloth as it picks up grime. Avoid soaking any electrical components.
5. Clean the evaporator coil fins if accessible. If you can reach the coil behind the filter housing, use a soft brush or a short burst of compressed air to gently dislodge dust and debris from the fins. Work carefully as the fins bend easily. Do not spray water directly onto the coil.
6. Check and clear the drain pan and drain line. Look for standing water or sludge in the drain pan. Wipe it out with a cloth and flush the drain line with a small amount of diluted vinegar solution to clear any partial blockages. A clear drain line means condensation exits the unit properly instead of pooling.
7. Allow everything to dry completely before switching back on. Leave the front panel open and give the unit at least 30 to 60 minutes to air out. Only reinsert the filter once it is fully dry. Then restore power and run the fan-only mode for 15 minutes before switching to cooling or heating.

When to Call a Professional Instead

DIY cleaning works well for surface mould on filters, vents and accessible panels. But there are situations where it simply isn't enough, and pushing ahead yourself can spread the problem rather than fix it.

Call a licensed technician if you notice any of the following:

• Mould is visible on the evaporator coil itself, not just the filter in front of it
• You have a ducted system and suspect mould inside the ductwork
• The mould has been present for a long time or covers a large area
• The musty smell persists even after a thorough DIY clean

A licensed technician has the equipment to do a deep coil clean using foaming coil cleaners and high-pressure tools that aren't practical for a homeowner. They can also sanitise the full system and inspect the drain line properly. A professional aircon service typically costs between $150 and $300 depending on the system type and the extent of the work. For a breakdown of what to expect, see our guide to air conditioner service prices in Australia. For serious mould infestations, that cost is well worth it compared to the alternative of breathing contaminated air or replacing a unit that's been left to deteriorate.

How to Prevent Mould in Your Aircon for Good

Preventing mould in your aircon comes down to controlling two things: moisture and maintenance. A few consistent habits are all it takes to keep mould from getting a foothold, and none of them require much time or money. The goal is to make your unit a less hospitable environment for mould, and that's easier than most people think.

The most effective prevention habits are:

• Clean or replace the air filter every 4 to 6 weeks during heavy use. A clogged filter restricts airflow, traps moisture and gives mould exactly what it needs to grow. This single habit makes more difference than anything else.
• Run fan-only mode for 15 to 20 minutes after cooling. After you finish a cooling session, switch the unit to fan-only before turning it off. This dries out the evaporator coil and internal surfaces, removing the residual moisture that mould feeds on. Some manufacturers call this the 'dry-out' function and build it in automatically.
• Keep the area around the indoor unit well-ventilated. Avoid blocking the unit with furniture or curtains. Good airflow around the unit helps moisture disperse rather than settle.
• Schedule a professional service once a year. An annual service catches early mould growth, clears the drain line and keeps the coil clean. It's the most reliable way to stay ahead of the problem.
• Use dry mode on humid days. Running dry mode reduces indoor humidity, which directly limits the moisture available for mould growth inside the unit. It's a simple habit that pays off over time.
• Consider a dehumidifier in particularly humid rooms. In rooms that stay muggy regardless of the aircon, a standalone dehumidifier takes the load off the unit and keeps ambient humidity at a level where mould struggles to grow. The Mitsubishi Electric Dehumidifier MJ-EV38HR-A ($1,385) is a premium option with a 38-litre-per-day extraction capacity, making it well suited to large or persistently humid spaces.

If you're upgrading your system, it's worth looking at models with built-in self-cleaning or auto-dry functions. The Daikin Cora 2.5kW Split System FTXV25WVMA ($1,039) is a good example. It has a washable filter and auto-restart, which reduces the chance of moisture sitting in the unit after a power interruption. For rooms where mould spores are already a concern, a standalone air purifier with a HEPA filter is a useful addition. The Daikin Air Purifier HEPA MC30YPVM ($359) is a compact option that captures airborne particles down to 0.3 microns, including mould spores. Browse our range of air purifiers to find one suited to your room size.

Does Dry Mode Help Prevent Mould?

Dry mode reduces indoor humidity by running the aircon's cooling cycle at a lower fan speed, which pulls more moisture out of the air without dropping the temperature as aggressively as full cooling mode. Lower humidity means less moisture settling inside the unit and on surrounding surfaces, which directly limits the conditions mould needs to establish itself. It's not a substitute for regular cleaning, but running dry mode on muggy days is a genuinely useful habit rather than just a comfort setting. Read our full explainer on dry mode on an air conditioner to understand when and how to use it effectively.

Thinking About Upgrading? Choose an Aircon That Fights Back Against Mould

If your unit is more than 10 years old, has been heavily mould-affected multiple times, or needs a professional clean every few months just to stay usable, an upgrade may actually cost less over time than repeated service calls. Modern split systems have come a long way on the mould front, with features like self-cleaning cycles, auto-dry functions and anti-bacterial filters built in as standard rather than extras.

The Daikin Cora 2.5kW Split System FTXV25WVMA ($1,039) is a good example of what a current-generation unit offers. It has a washable filter and an auto-restart function, which means the unit resumes its previous settings after a power interruption rather than sitting idle with residual moisture on the coil. Small features like these make a real difference to how often mould gets a chance to establish itself.

If you're not ready to replace the unit but want to improve air quality in a mould-prone room, a standalone air purifier is worth considering. The Daikin Air Purifier HEPA MC30YPVM ($359) captures airborne particles down to 0.3 microns, including mould spores, and works independently of your aircon. It's a practical option for bedrooms or living areas where mould has been a recurring issue.

Browse our full range of split system air conditioners to compare models with built-in anti-mould features.

Frequently Asked Questions About Mould in Aircon

Is mould in an aircon dangerous?

Yes, mould in an aircon can pose real health risks. The unit circulates air through the affected components, spreading spores into every room it serves. This can trigger allergic reactions, worsen asthma and cause respiratory irritation, particularly in children, the elderly and anyone with a compromised immune system. The longer mould is left untreated, the greater the exposure risk.

What causes mould to grow in an aircon?

Mould grows inside aircon units because the internal environment is consistently damp, dark and warm, exactly the conditions mould needs. Condensation forms on the evaporator coil during normal operation, and if the unit isn't dried out properly after use, that moisture lingers. Dust and debris on the filter give mould an organic surface to feed on, and poor ventilation or high ambient humidity speeds the process up considerably.

Can I run my aircon if it has mould?

Running a mould-affected aircon will spread spores through your home, so it's best to address the problem before continuing normal use. If the mould is limited to the filter, remove and clean the filter first, then run the unit on fan-only mode briefly to help dry the internals. For anything beyond surface mould on the filter, stop using the unit and arrange a professional clean before switching it back on.

How often should I clean my aircon to prevent mould?

Clean or rinse the air filter every four to six weeks during periods of heavy use, and run fan-only mode for 15 to 20 minutes after each cooling session to dry out the coil. A full professional service once a year is the most reliable way to keep mould from taking hold in the parts you can't easily reach yourself, such as the evaporator coil and drain pan.

Keep Your Aircon Clean and Your Air Healthy

Mould in an aircon is common across Australia, but it's far from inevitable. Three habits cover most of the risk: inspect your filter and vents regularly, clean promptly when you spot the early signs and build a few simple prevention routines into your normal use. Stay on top of those three things and mould rarely gets a chance to become a serious problem.

If your unit is ageing or has been a recurring mould battleground, it may be time to consider a fresher start. Modern split systems and standalone air purifiers give you better tools to keep the air clean from day one.

Have questions about which system suits your home? Get in touch with the team at AusAir Conditioners for honest, no-pressure advice.