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Why does mould keep coming back in my bathroom?

The short answer

Mould returns because the moisture source is still there. Cleaning treats the symptom, and unless ventilation, a leak or a cold surface is addressed, it will regrow in the same place.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 4 published sources, listed in full at the end of this page. How we source this

Mould needs moisture, a surface and time. Removing it without removing the moisture guarantees it returns, usually in the same spot, which is the pattern that tells you the cause has never been addressed.

Work out which moisture source it is

  • Ventilation. If it appears on ceilings and upper walls generally, and worsens in winter, it is condensation from inadequate extraction.
  • A leak. If it is concentrated in one place, particularly low on a wall, at a skirting or on the other side of a shower wall, suspect water rather than air.
  • A cold bridge. Mould in a single corner or along a specific line often marks a spot where the surface is colder than everything around it.
  • Grout and silicone failure. Deteriorated joints let water into the substrate, which then stays wet.

The order to fix it in

  1. Confirm the exhaust fan runs, moves real air, and discharges outside rather than into the roof.
  2. Run the fan during the shower and for a good period afterwards, which is when most of the moisture leaves.
  3. Check and renew failed silicone at wall junctions and around the floor waste.
  4. If it is localised and persistent, investigate for a leak before spending anything on cosmetic work.

Who does this work

You may need a registered builder

Because cleaning and ventilation you can do yourself, but rewiring a fan to vent outside is licensed electrical work and a failed membrane is building work.

Depending on the cause it can also involve electrician or waterproofer.

This work is licensed, though the threshold and the licence class differ between states. Check a licence in your state

What to do now

  1. Run the exhaust fan and hold a sheet of paper to it. If the paper does not hold, the fan is not moving enough air to matter.
  2. Find where the fan discharges. A fan venting into the roof space moves the moisture rather than removing it, and it is a very common fault.
  3. Run the fan for fifteen to twenty minutes after showering, not just during.
  4. If the mould is at floor level or on the far side of a shower wall, treat it as a possible waterproofing failure rather than a ventilation problem.

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. Guidance material and handbooks · Australian Building Codes BoardChecked 2026-08-11 · responded 200 OK
  2. National Construction Code · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  3. Building and Plumbing Commission · Building and Plumbing Commission VictoriaChecked 2026-08-10 · live, but blocks automated clients so the specific page could not be confirmed
  4. Home Energy Rating (NatHERS) · Australian GovernmentChecked 2026-08-10 · responded 200 OK

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