Smelly Drains: Causes & How to Fix the Odour

Published 2026-06-02 · Rotorua Plumbers

What causes a smelly drain

A healthy drainage system is sealed against sewer gas by water-filled traps. When that seal fails — or waste builds up — odours escape. The usual causes are:

• A dry P-trap — the U-bend under a sink or in a floor waste holds water that blocks sewer gas. In a rarely-used shower, laundry tub or guest bathroom, that water evaporates and the smell comes straight up.
• Blockages and biofilm — fat, hair, soap and food scraps build up into a slimy biofilm that rots and stinks long before it fully blocks the pipe.
• Gully traps — the outside gully can smell if its water seal dries out, it's clogged with leaves, or it's been overgrown or covered.
• Venting issues — drains need air vents to flow properly. A blocked or faulty vent can siphon water out of traps, breaking the seal and letting gas back inside.

DIY first steps

Start simple. Run water into any drain you don't use often — a litre or two refills a dry trap and the smell usually disappears within minutes. Clean the plughole and overflow, and clear hair and gunk from the strainer. For biofilm, flush with hot (not boiling) water and a baking-soda-and-vinegar wash, or a drain-safe cleaner. Outside, clear leaves and debris from the gully trap and make sure its grate isn't buried.

Rotorua's sulphur smell vs a drain fault

Worth saying clearly: Rotorua's natural geothermal “rotten egg” smell in the air and sometimes the hot water is hydrogen sulphide from the ground — that's a different thing entirely from a drainage odour. If the smell is localised to one drain, comes and goes with use, or is paired with slow draining, it's a plumbing issue, not the geothermal field.

When the smell signals a real fault

If refilling traps and a clean-out don't fix it, the odour points to something deeper: a partial blockage, a cracked or displaced pipe letting gas escape, a failed gully, or a venting fault. Persistent smells, gurgling drains, slow draining across several fixtures, or sewer odour through the house are all worth a proper look. We can camera the line and trace the source rather than just masking it.

How we can help

• Blocked drains, gully traps & CCTV
• Drainage repairs & 24/7 emergency