Black Mould in Your Air Conditioner: Causes & Fixes

Illawarra Mould Removal treats mould inside an air conditioner or split system as a condensation problem, not just a cleaning one: warm, humid coastal air meets a cold coil and drain tray, and a filter or coil left damp between cycles grows mould, producing that musty blast of air on startup. A dusty filter is usually DIY; smell that keeps returning generally needs a technician or a proper inspection.

That short version covers most cases, but “usually DIY” and “needs a professional” sit either side of a line that’s easy to misjudge with an appliance you can’t fully see inside. Here’s how to tell which side of it you’re on.

Why Does My Air Conditioner Smell Musty When I Turn It On?

Because air passing over a damp, mould-affected filter or coil picks up that earthy, cardboard-like odour and carries it straight into the room, usually strongest in the first minute or two after startup and fading as airflow clears it. This is one of the most common musty-smell patterns we hear about across Wollongong, Shellharbour and Kiama homes, and it’s specific enough to diagnose with reasonable confidence without opening the unit: if the smell is worst right on startup and eases off, the unit itself is the source far more often than a subfloor or roof void. Our broader guide to musty smells in Illawarra homes covers the other hidden-cavity causes if the pattern doesn’t match this one, for example if the smell is just as strong with the aircon off, or strongest at floor or ceiling level rather than at the vents.

The reverse is also a useful check: if the unit runs clean and odourless but the room still smells musty once it’s off, the aircon probably isn’t your source, and it’s worth looking at ventilation, a nearby wardrobe, or the subfloor instead.

What Causes Black Mould Inside a Split System in the Illawarra?

Condensation, almost every time. A split system’s indoor coil runs cold enough to pull moisture out of the air, and that water is meant to drain away through the tray and drain line. Mould gets a foothold when:

• The unit sits idle with residual moisture. A coil and drain tray that stay damp between cooling cycles, especially in a closed-up room, never fully dry, and organic dust on the filter gives mould something to feed on.
• The filter is overdue for a clean. Dust, skin cells and general household grime build up on the filter surface, and combined with the coil’s condensation, it’s a reliable growth medium.
• The drain line is partially blocked. Algae, dust and mineral build-up can slow drainage, leaving water sitting in the tray longer than it should.
• The unit is oversized or short-cycling. A system that cools the room quickly then shuts off repeats the “cold, damp, then stagnant” cycle more often than a correctly sized one running longer, steadier cycles.
• The room itself runs humid. Illawarra winters routinely push closed-up indoor air above roughly 70 per cent relative humidity without active ventilation, well above the 60 per cent ceiling (ideally 40-55 per cent) that keeps sustained mould growth in check. A split system working inside that kind of air has more moisture to deal with than one in a well-ventilated room.

None of this is unique to any one suburb; it’s a function of running refrigerated cooling in a climate that keeps ambient humidity high for much of the year, which is also why bathroom and bedroom ceilings in the same homes tend to grow mould for closely related reasons. If you’re trying to work out whether what you’re looking at is genuinely a problem or just discolouration, our black mould facts and myths guide is worth a read before you do anything drastic to the unit.

Can I Clean Air Conditioner Mould Myself, or Do I Need a Professional?

It depends on where the mould is and how it behaves after cleaning. As a general rule:

DIY is reasonable when:

1. The mould is limited to the filter, which is designed to be removed, washed and dried
2. It’s a first occurrence, not a repeat visitor after cleaning
3. You can see and reach the affected part without opening the unit’s casing
4. There’s no unusual smell coming from inside the wall or ceiling around the unit

Get help (an air conditioning technician, and possibly a mould inspection) when:

1. The smell or growth returns within weeks of a filter clean
2. You can see mould on the coil, fan blades or inside the housing, none of which are meant to be opened or handled by an untrained person
3. There’s visible mould or staining on the ceiling or wall around or below the unit, not just inside it
4. The unit has an active drip or drain problem, rather than just a smell
5. It’s a ducted system, where the affected area (coils, ducting) usually isn’t accessible without proper equipment

Filters on most split systems simply slide or clip out. Washing a filter in mild detergent, rinsing it well and letting it dry completely before refitting handles a genuinely dusty, first-time case. What a filter wash won’t fix is mould that’s colonised the coil itself, or a drain line that’s slowing drainage: both sit past the point where opening the unit further risks damaging it or handling a refrigerant-containing system you’re not licensed to work on.

Filter Clean vs Professional Service vs a Mould Inspection: What’s the Difference?

These three responses solve different problems, and picking the wrong one is a common way people spend money without actually fixing the smell. The table below sets out what each is for, based on the pattern you’re seeing.

*Indicative and region-general only. Every figure depends on the actual job; the two mould-related rows are confirmed by photo assessment, inspection or a formal quote, the same as every other service on this site.

When Does Aircon Mould Become a Job for a Mould Removal Specialist Rather Than an Aircon Technician?

The moment the problem stops being contained inside the appliance. Cleaning or servicing the unit itself, including anything involving the sealed refrigerant circuit, is aircon technician territory: in Australia, working on a system’s refrigerant components requires licensing administered by the Australian Refrigeration Council, which is a genuinely different trade and licensing regime to mould remediation. Illawarra Mould Removal’s partner network doesn’t service air conditioners.

Where our side of things comes in is the building envelope around the unit: if condensation dripping from a poorly maintained system, or a blocked drain overflowing, has left mould or a musty smell on the surrounding ceiling, wall, or in a wardrobe sharing that wall, that’s a building-surface problem our partner network of qualified, licensed local remediation professionals handles. It’s also worth booking a mould inspection and moisture investigation if you’ve had the aircon serviced, the appliance smell is gone, but the room still smells musty: that points to a separate moisture source that servicing the unit was never going to fix.

Is Mould in My Air Conditioner Making My Family Sick?

We’re a building-remediation business, not a medical authority, so we don’t make health claims and won’t guess at symptoms. What we can say factually is that health authorities, including NSW Health, recommend addressing mould and damp in the home generally, regardless of where it’s growing or what colour it is. If anyone in your household has health concerns they suspect are related to the home environment, that’s a conversation for a GP, not a website. Our role is finding and fixing the moisture side of the problem.

How Do I Stop Aircon Mould Coming Back?

Treat the cycle, not just the smell, with habits that address both the appliance and the room it’s in:

1. Clean or replace the filter on the manufacturer’s schedule, more often in a bedroom used nightly or a home near the coast.
2. Run the fan-only mode for 10-15 minutes after cooling, if your unit has it, so residual condensation on the coil dries out instead of sitting overnight.
3. Keep the room’s humidity in check. The same target that applies everywhere else in an Illawarra home applies here: below roughly 60 per cent, ideally 40-55 per cent, using cross-ventilation on dry days and by not overcrowding a small, closed room with the unit running constantly.
4. Get the drain line checked if you’re due for a service and haven’t had a clean in a year or two; a slow drain is invisible until it isn’t.
5. Watch the wall around the unit. A quick visual check every season for staining, bubbling paint or a new musty note catches a spreading problem while it’s still small.

If you do all of that and the smell still comes back, the honest next step is to stop guessing and get it looked at properly. Get a free quote and describe the pattern (on startup, constant, worse after rain) along with your suburb, and we’ll give you a straight read on whether it’s a servicing issue, a building issue, or worth a proper inspection.

Air Conditioner Mould FAQs

Why does my air conditioner smell musty when I first turn it on?

Because air moving over a damp, mould-affected filter or coil carries that earthy odour into the room, typically strongest in the first minute or two and fading as fresh airflow clears it. It’s one of the more identifiable musty-smell patterns because of how closely it tracks with the unit switching on.

Can I just wash the filter myself?

Yes, for a straightforward, first-time case. Remove the filter as per the manual, wash it in mild detergent, rinse thoroughly and let it dry completely before refitting. If the smell or visible mould returns within weeks, the problem has likely moved past the filter to the coil or drain tray, which is a job for a licensed technician.

Is it safe to open up the unit and clean the coil myself?

We’d recommend against it. Split systems contain a sealed refrigerant circuit that legally requires a licensed technician (licensing is administered nationally by the Australian Refrigeration Council) to work on, and incorrectly handled coil cleaning can damage the unit or void a warranty. Filter cleaning is the safe DIY boundary; anything past that is a technician’s job.

Does mould in my aircon mean my whole house has a mould problem?

Not necessarily. Aircon mould is often a contained, appliance-level condensation issue that clears up with a filter clean or professional service. It’s worth wider investigation only if the musty smell persists with the unit off, or you notice mould on the ceiling or wall around the unit, either of which points to a building-related moisture source rather than just the appliance.

Who do I call: an air conditioning technician or a mould removal service?

Start with an aircon technician for anything inside the unit itself, filter, coil, drain line or refrigerant components. Call a mould removal service when the mould or musty smell has spread to the surrounding ceiling, wall or a nearby wardrobe, or when you can’t tell whether the aircon is really the source. If you’re unsure which applies, send us photos and we’ll tell you honestly which one you need.

Will a dehumidifier fix mould in my air conditioner?

Not directly. A portable dehumidifier can help lower general room humidity, which reduces how much moisture the unit has to deal with, but it won’t reach mould already established on a filter or coil. That still needs a physical clean, DIY for the filter or professional for anything deeper.