How Do I Unblock a Drain That Has a Foul Smell and Slow Flow?

A slow drain with a foul smell is one of the most common household complaints in New Zealand — and in most cases, it's something you can tackle yourself with a bit of patience and the right approach. The smell usually comes from decomposing organic matter (hair, grease, food scraps, or soap scum) trapped in the pipe or sitting in the drain trap, while the slow flow tells you there's a partial or full blockage restricting water movement.

Identify Where the Problem Is

Before reaching for the drain cleaner, work out which drain is affected and whether the problem is isolated or widespread.

• Single fixture slow/smelly (e.g. just the bathroom basin): likely a localised blockage in that drain or its P-trap.
• Multiple fixtures backing up at once: could indicate a blockage further down the line in a shared branch drain or even the main sewer drain.
• Gurgling sounds from other fixtures: a sign of a venting issue or a deeper blockage.

If multiple drains are affected or you notice sewage backing up into a floor waste or toilet, stop and call a licensed professional — this is beyond a DIY fix.

Step 1 — Remove Visible Debris

Start simple. Remove the drain cover or plug and use gloved hands or a pair of needle-nose pliers to pull out any hair, soap build-up, or gunk sitting near the surface. You'd be surprised how often this alone restores full flow. Clean the drain cover with an old brush and hot soapy water.

Step 2 — Flush With Boiling Water (With Caution)

For kitchen sinks and floor wastes with grease build-up, carefully pour a full kettle of boiling water slowly down the drain in two or three stages, allowing it to work between pours. This can melt and shift grease blockages.

Caution: Do not use boiling water on PVC plastic pipes or older ceramic fixtures, as it can cause damage or cracking. Use very hot (but not boiling) water instead if you're unsure of your pipe material.

Step 3 — Baking Soda and Vinegar Method

This is a safe, eco-friendly option that works well for mild blockages and odours:

1. Pour ½ cup of baking soda down the drain.
2. Follow with ½ cup of white vinegar.
3. Cover the drain opening and leave for 15–30 minutes.
4. Flush through with hot water.

The fizzing reaction helps break up light organic build-up and neutralises odours. It won't shift a serious blockage, but it's a great first step and safe for all pipe types.

Step 4 — Use a Plunger

A cup plunger (the standard rubber one) works well on basins, baths, and laundry tubs. Block the overflow hole with a damp cloth to create proper suction, then plunge firmly 10–15 times. For toilets, use a flange plunger designed for that purpose.

Step 5 — Try a Drain Snake or Hair Clog Remover

A hand-operated drain snake (also called an auger) or a cheap plastic hair clog remover tool (available at Bunnings or Mitre 10) can reach deeper into the pipe to hook and pull out blockages that plunging won't shift. Feed it gently into the drain, rotate, and pull back slowly.

Step 6 — Chemical Drain Cleaners (Use Sparingly)

Chemical drain cleaners are available at most hardware stores but should be used as a last resort before calling a professional. Follow the manufacturer's instructions carefully and:

• Never mix different chemical products — this can cause dangerous reactions.
• Wear gloves and eye protection.
• Ensure the area is well ventilated.
• Avoid repeated use on older pipes, as some formulas can degrade seals and fittings over time.

Check WorkSafe NZ guidelines if you're handling any hazardous chemical products.

Dealing With the Foul Smell

If the drain flows freely but still smells, the culprit is often a dry P-trap — the curved section of pipe beneath the fixture that normally holds a small amount of water to block sewer gases. If a drain hasn't been used for a while (e.g. a spare bathroom or laundry tub), simply run the water for 30 seconds to refill the trap.

A persistent sewer smell throughout the house may indicate a broken or missing trap, a venting problem, or a cracked drain pipe — all of which require a licensed professional to assess.

When to Call a Professional

Some drain issues are beyond DIY and require a licensed drainlayer or registered plumber. Under New Zealand law, drainlaying work (work on pipes that carry waste to the sewer or septic system) is restricted work and must be carried out by someone registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).

Call a professional if:

• Multiple drains are blocked or backing up simultaneously.
• You suspect a broken, collapsed, or tree-root-invaded drain pipe.
• There is sewage odour throughout the home that doesn't clear.
• The blockage keeps returning despite repeated DIY attempts.
• You need to clear or repair any underground drain line.

You can find qualified, local drain cleaners and plumbers on tradedirectory.co.nz.

For guidance on what work requires consent or a licensed tradesperson, visit MBIE Building Performance.

Sources & Further Reading

• Plumbers, Gasfitters and Drainlayers Board — Licensed Practitioners
• MBIE Building Performance — Building Work
• WorkSafe NZ — Hazardous Substances
• BRANZ — Home Maintenance Guidance
• Bunnings NZ — Drain Cleaning Tools
• Mitre 10 NZ — Plumbing Supplies