Mould needs two things to grow indoors: moisture and time. Every mouldy bathroom ceiling, wardrobe wall and window sill in Australia comes back to that pair, which is also why scrubbing mould off without fixing the moisture guarantees a rematch in a few weeks.
This guide explains why it grows where it does, how to remove small patches safely, when the job genuinely needs professionals, and who is responsible for what in a rental.
Why bathrooms, bedrooms and wardrobes grow mould
Bathrooms are the obvious case: showers put litres of water vapour into the air daily, and if the exhaust fan is weak, blocked or unused, that moisture condenses on the coldest surfaces, usually the ceiling and the grout, and mould follows.
Bedrooms and wardrobes surprise people, but the mechanism is the same. Sleeping people exhale moisture all night into a closed room; it condenses on cold external walls and windows, particularly in winter and particularly behind furniture pushed hard against those walls, where air cannot circulate. Wardrobes on external walls are worst of all: still air, stored fabric and a cold surface. South-facing rooms, homes without eaves and buildings with poor insulation all run colder wall surfaces and grow more mould for the same indoor moisture.
Safe DIY removal for small patches
For small areas, a patch smaller than about a square metre on a non-porous or semi-porous surface, DIY removal is reasonable. Australian health guidance generally recommends a vinegar solution over bleach for mould on porous surfaces: mix roughly four parts white vinegar to one part water, apply to the mould, leave it to sit for up to an hour, then wipe off with a damp cloth and dry the surface.
The reason vinegar beats bleach on porous surfaces like plasterboard, grout and timber is that bleach mostly strips the colour from surface growth while leaving the root structure alive within the material, so the mould appears gone and returns. Bleach also masks regrowth by whitening the area. On hard non-porous surfaces like glass and tile faces, detergent and water followed by thorough drying does the job.
- Wear gloves, eye protection and a P2 mask; mould spores are an irritant even for healthy people
- Never mix vinegar or any cleaner with bleach; the combinations produce hazardous gases
- Microfibre cloths work best; rinse them frequently or bin them afterwards rather than spreading spores
- Dry the area thoroughly after cleaning; a fan or open window for a few hours finishes the job properly
- Anyone with asthma, allergies or a compromised immune system should not do mould removal themselves
When to call professionals
Some situations are past DIY regardless of enthusiasm. Large affected areas, as a common benchmark anything much beyond a square metre, mould inside wall cavities, carpets or ceiling insulation, growth that returns quickly after proper cleaning, and any mould following a flood, storm damage or a plumbing leak all justify professional remediation.
Professional remediators do more than clean harder. They find the moisture source with meters and thermal cameras, contain the work area so spores do not spread through the house, remove materials that cannot be saved such as soaked plasterboard and underlay, and dry the structure with commercial dehumidifiers before anything is rebuilt. If mould keeps returning to the same spot after correct cleaning, the message is that a moisture source is active, often a slow leak, rising damp or condensation from a design flaw, and finding it matters more than the next scrub.
Prevention: manage moisture, not mould
Every effective prevention step is a moisture step. Mould cannot be argued with, but it can be starved.
- Run bathroom exhaust fans during every shower and for 10 to 15 minutes after, and clean the fan grille so it can actually move air
- Use the rangehood when cooking, and vent clothes dryers outside or run them with a window open
- Dry clothes outdoors or in a vented space; an indoor drying rack releases litres of water into the room
- Open windows for a burst of cross-ventilation daily, especially in winter when homes are sealed tight
- Keep furniture a hand's width off external walls and leave wardrobe doors ajar occasionally
- In persistently damp rooms, a dehumidifier holding humidity under about 60 percent is the single most effective appliance
- Fix dripping taps, leaking showers and blocked gutters promptly; small chronic leaks feed most stubborn mould
Rentals: who is responsible for mould?
In general terms across Australian tenancy law, responsibility follows cause. Mould arising from the property itself, a leaking roof, failed waterproofing, rising damp, no functioning exhaust fan, is the landlord's to fix, and tenants should report it in writing as soon as it appears. Mould arising from tenant behaviour, never ventilating, drying laundry indoors daily with windows shut, is the tenant's to manage.
In practice many cases involve both, which is why the written report matters: it starts the clock on the landlord's obligation to maintain the property in reasonable repair and protects the tenant at bond time. Rules and remedies differ by state and territory, so for a dispute, the state tenancy authority is the right source rather than general advice. Meanwhile, whoever is responsible for the cause, ventilating well and reporting leaks early serves everyone in the property.
Frequently asked questions
Is household mould dangerous?+
For most healthy people small amounts cause irritation at worst, but mould can trigger asthma, allergic reactions and respiratory symptoms, and the risk rises for children, the elderly and anyone immunocompromised. Persistent indoor mould is a health issue worth fixing properly, not just a cosmetic one.
Why does mould keep coming back after I clean it?+
Because cleaning removes the growth, not the cause. Regrowth means moisture is still arriving faster than the surface can dry: condensation, a leak or dead air behind furniture. Fix the ventilation or the leak and cleaning becomes a once-off; skip that step and it is a subscription.
Should I just paint over a mouldy ceiling?+
No. Paint, even so-called anti-mould paint, over live mould traps the problem and it returns through the new coat. Clean the mould off, fix the moisture source, let the surface dry fully, then repaint, ideally with a mould-inhibiting paint in wet areas. Painting over mould in a rental can also read as concealment at inspection time.
Can a regular cleaner deal with mould?+
A cleaner can remove light surface mould from bathroom tiles and grout as part of a normal visit. Established patches on ceilings and walls, anything inside materials, and recurring growth belong to specialist mould remediation, which is a different trade with containment and drying equipment. Ask before assuming it is in scope.