The overflow hole in your bathroom sink is a small but important feature — it prevents the basin from flooding if the tap is left running. Unfortunately, it's also a prime spot for biofilm, soap scum, hair, and mould to build up, causing unpleasant smells and, eventually, sluggish drainage. The good news is that cleaning it is a straightforward DIY job that most homeowners can tackle in under 30 minutes.
What Is the Overflow Hole and Why Does It Get Dirty?
The overflow hole is the small opening near the top of your basin, usually positioned just below the rim. It connects via a hidden internal channel to the main drain. Because this channel is dark, damp, and rarely flushed through, it becomes a breeding ground for biofilm — a slimy layer of bacteria, soap residue, and organic matter. Over time this is the most common cause of that persistent musty or rotten-egg smell coming from your bathroom sink, even when the drain itself seems clear.
What You'll Need
- Small bottle brush or flexible pipe brush (available at Bunnings or Mitre 10)
- Old toothbrush
- White vinegar
- Baking soda
- Mild dish soap
- A small funnel (optional but handy)
- Rubber gloves
- A torch or phone light
Avoid using harsh chemical drain cleaners (such as caustic soda or strong acid-based products) in the overflow hole. These can damage older chrome or plastic fittings and are difficult to flush out of the narrow internal channel safely.
Step-by-Step: How to Clean the Overflow Hole
Step 1 – Inspect the Hole
Put on your rubber gloves and use a torch to peer into the overflow hole. You'll likely see a dark, slimy build-up. This confirms biofilm is the culprit.
Step 2 – Dry Brush First
Insert a small bottle brush or flexible pipe brush into the overflow hole and scrub in a circular motion. Work the brush as far into the channel as it will comfortably reach. This loosens the bulk of the biofilm and debris before you apply any liquid.
Step 3 – Apply Baking Soda
Using a small funnel or a folded piece of paper, pour roughly 2–3 tablespoons of baking soda into the overflow hole. Let it sit for a couple of minutes.
Step 4 – Flush with White Vinegar
Slowly pour half a cup of white vinegar into the overflow hole. The fizzing reaction helps lift and break down soap scum, grease, and organic matter inside the channel. Leave it to work for 10–15 minutes.
Step 5 – Scrub Again
Insert the bottle brush again and give the channel another thorough scrub while the vinegar solution is still active.
Step 6 – Flush with Hot Water
Boil the kettle and allow the water to cool slightly (very boiling water can crack some ceramic basins — aim for around 60–70°C). Pour the hot water slowly into the overflow hole to flush everything through into the main drain. Follow up by running the hot tap for a minute or two.
Step 7 – Clean the Exterior Opening
Use an old toothbrush dipped in a little dish soap to scrub around the outside rim of the overflow hole, removing any visible grime or mould from the basin surface.
How Often Should You Do This?
| Situation | Recommended Frequency |
|---|---|
| Single-person household | Every 3–4 months |
| Family home with heavy use | Every 4–6 weeks |
| Rental property (Healthy Homes) | As required to maintain drainage in good working order |
| After noticing odours | Immediately |
For rental properties, landlords should be aware of their obligations under the Healthy Homes Standards, which include maintaining drainage and ventilation in good working order.
Preventing Build-Up Between Cleans
- Once a fortnight, pour a cup of hot water mixed with a small squirt of dish soap into the overflow hole.
- Keep a small bottle brush under the sink as a reminder to clean the overflow when you clean the basin.
- Ensure your bathroom has adequate ventilation — a good extractor fan reduces the humidity that encourages biofilm growth. Energywise has useful guidance on ventilation in NZ homes.
When the Smell Persists After Cleaning
If odours continue after a thorough clean, the problem may lie deeper in the drain trap or waste pipe rather than the overflow channel itself. A partially blocked P-trap or a dry trap (common in infrequently used basins) can produce similar smells. In this case, check the P-trap under the sink — this is generally a DIY-accessible task, as it involves unscrewing the trap and clearing any blockage.
However, if you suspect the issue is further down the drain line, or if you notice slow drainage throughout multiple fixtures, the blockage may be in the branch drain or main drain stack, which requires a professional assessment.
When to Call a Professional
Call a qualified drain cleaner or plumber if:
- Odours persist after cleaning the overflow hole and clearing the P-trap
- Water is backing up into the basin or other fixtures
- You notice gurgling sounds from the drain, which can indicate a venting issue
- The overflow hole or waste fitting appears cracked or damaged and needs replacing
Replacing waste fittings that connect to the drain system is restricted plumbing work under the Plumbers, Gasfitters, and Drainlayers Act 2006 and must be carried out by a licensed tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Always ask to see a tradesperson's current licence before work begins.
You can find qualified, local drain cleaners and plumbers on tradedirectory.co.nz.
Master Plumbers
Certifying Plumber
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