Every drain opening in your home is protected by a water trap — the U-shaped bend holding a plug of water that stops sewer gas coming back up. When a bathroom smells of sewage, one of those seals has usually failed.
Work through the likely causes:
- Dried-out floor waste. The floor drain in a bathroom that rarely gets water (guest bathrooms, ensuites with shower trays plumbed separately) loses its trap water to evaporation — in Brisbane heat, sometimes within weeks. Fix: pour a bucket of water down it. If the smell fades over a day, that was it. A dash of vegetable oil on top slows future evaporation.
- Siphoned trap. Gurgling plus smell suggests a partial blockage or vent problem is sucking traps dry each time water drains — the trap empties itself.
- Biofilm and gunk. A slimy build-up of soap, hair and bacteria in the waste itself smells distinctly "drainy". Cleaning the waste and trap fixes this one.
- Failed toilet pan seal. The rubber or wax seal between toilet and drain can perish, leaking gas (and sometimes water) at floor level. A toilet that rocks even slightly is a suspect.
- Blocked or inadequate vent. A blocked roof vent disturbs the pressure balance and can pull traps dry throughout the house.
If refilling traps and cleaning wastes does not solve it, or the smell comes with gurgling and slow drains, the cause is in the drainage system itself and worth professional diagnosis. Persistent sewer gas is unpleasant and, in confined spaces, a health matter — do not simply mask it with air freshener.