Bathroom mould is one of the most common household problems in Australia, particularly in homes with poor ventilation or in coastal and humid climates. If you catch it early and treat it correctly, most surface mould can be managed at home. If it has been growing for a while, or keeps coming back despite treatment, there may be an underlying cause worth addressing.
Is It Actually Mould?
Before treating anything, it helps to know what you are dealing with. Mould in bathrooms typically appears as black, green, or grey spots on grout lines, silicone seals, ceiling corners, and around window frames. It often has a slightly fuzzy or raised surface when looked at closely.
Pink or orange residue around shower drains and tile surfaces is usually bacterial biofilm (Serratia marcescens), not mould. It responds to similar treatments but is easier to remove.
A flat grey or black stain that does not respond to surface cleaning may indicate mould growth behind tiles or inside the wall cavity, which is a different situation.
Safety First
Mould spores can cause respiratory irritation. Before cleaning:
- Open windows and run the exhaust fan
- Wear rubber gloves
- A basic dust mask is helpful if the affected area is large
- Avoid mixing cleaning products, particularly bleach with anything containing ammonia
Surface Mould: What Actually Works
Bleach-Based Solution
A diluted bleach solution is the most effective readily available treatment for surface mould on non-porous materials like tiles, grout, and silicone.
Mix one part household bleach with four parts water in a spray bottle. Apply to the affected area, leave for 10 to 15 minutes, then scrub with a stiff brush and rinse thoroughly. For grout lines, an old toothbrush gives better access than a standard sponge.
Do not use bleach on natural stone tiles such as marble, travertine, or limestone. It causes discolouration and surface damage. Use a pH-neutral stone cleaner instead.
Hydrogen Peroxide
Three percent hydrogen peroxide (available at pharmacies) is effective on mould and is less harsh than bleach. Apply directly, leave for 10 minutes, scrub, and rinse. Useful for surfaces where bleach is not appropriate or where you want to avoid strong fumes.
White Vinegar
Undiluted white vinegar kills many types of mould and is safe on most surfaces. Spray directly on the mould, leave for at least one hour, then scrub and wipe clean. It is less immediately effective than bleach for established growth, but a useful maintenance option.
Stubborn Areas: Silicone and Grout
Mould that has penetrated silicone sealant around the bath, shower base, or tap fittings often cannot be fully removed. The silicone will look clean on the surface but the mould continues to grow internally and returns within weeks.
In this situation the correct fix is removing the old silicone entirely, cleaning and drying the underlying surface, and applying fresh silicone. This is a straightforward job with the right tools: a silicone removal tool or utility knife, silicone remover solution, and a new cartridge of bathroom-grade silicone.
Discoloured grout that does not respond to scrubbing may need re-grouting, or can sometimes be restored with a grout pen as a cosmetic fix.
Why It Keeps Coming Back
Mould grows where moisture stays on surfaces. If you treat it and it returns within a few weeks, the problem is almost always ventilation.
Common causes:
- Exhaust fans that vent into the ceiling cavity rather than outside (check where your exhaust fan actually exhausts to)
- Exhaust fans that are undersized for the bathroom volume
- Exhaust fans that are not being run long enough after showers (10 to 15 minutes after finishing is a reasonable guide)
- Windows kept closed during and after showering
Improving airflow is the only lasting solution. Surface treatments address the visible growth but do nothing about the conditions that created it.
Ceiling Mould
Mould on bathroom ceilings is very common. Treat it with the same diluted bleach solution using a sponge on a long handle, or a spray bottle and a stepladder. After cleaning and allowing the surface to dry fully, applying a mould-inhibiting bathroom ceiling paint can help slow regrowth.
Tip: If the ceiling mould extends across a large area or appears to be staining from above rather than from condensation, it may indicate a roof or pipe leak. Worth investigating before painting over it.
When to Call a Professional
A professional clean is appropriate when:
- The affected area is large (more than one square metre is often cited as a practical threshold)
- Mould appears to have grown behind tiles or inside wall cavities
- You have tried surface treatment multiple times and it returns quickly despite addressing ventilation
- Someone in the household has respiratory conditions
For end-of-lease situations, active mould at inspection is likely to be noted and can result in bond deductions if it is considered the result of inadequate cleaning or ventilation during the tenancy.