A toilet that keeps running after flushing is one of the most common household plumbing complaints in New Zealand — and one of the most wasteful. A continuously running toilet can waste tens of thousands of litres of water per year, adding significantly to your water bill (if you're on a metered supply) and putting unnecessary strain on the environment. The good news is that most causes are straightforward to diagnose, and many can be fixed by a confident DIYer without calling a plumber.
Why Does a Toilet Keep Running?
Inside your toilet cistern (the tank at the back), there are a few key components that control filling and flushing. When any of these wear out or go out of adjustment, water keeps trickling into the bowl or overflowing into the overflow tube. The three most common culprits are:
- A faulty flapper valve – the rubber seal at the bottom of the cistern that lifts when you flush and drops to seal the cistern as it refills.
- A worn or misadjusted fill valve (ballcock) – the mechanism that controls water flowing into the cistern from the supply line.
- An incorrect float level – if the float is set too high, water spills into the overflow tube continuously.
Older NZ homes often have traditional ballcock-style fill valves, while newer toilets use modern float-cup fill valves. Both can fail over time due to mineral deposits (common in areas with hard water), UV degradation, or simple wear.
Common Symptoms and What They Mean
| Symptom | Likely Cause |
|---|---|
| Constant hissing or trickling sound | Faulty flapper or fill valve |
| Water running into bowl between flushes | Flapper not sealing properly |
| Cistern never fully fills | Fill valve issue or supply pressure problem |
| Water level too high, spilling into overflow | Float set too high |
| Toilet runs intermittently ("phantom flush") | Slow flapper leak |
How to Diagnose the Problem Step by Step
Step 1: Remove the Cistern Lid and Observe
Lift the lid off the cistern and set it safely aside. Watch what's happening inside. Is the water level sitting above the top of the overflow tube? Is water trickling down into the bowl? Is the fill valve running non-stop?
Step 2: Check the Float Level
The water level should sit roughly 25 mm below the top of the overflow tube. If it's level with or above the tube, water is constantly draining away. On a ballcock valve, gently bend the float arm downward slightly. On a modern float-cup valve, there's usually an adjustment screw or clip on the side of the fill valve.
Step 3: Test the Flapper
Push down firmly on the flapper with your finger while the toilet is running. If the running stops immediately, the flapper isn't sealing — it's either warped, worn, or has mineral build-up on the seat. Turn off the water supply valve (the tap behind or below the toilet), flush to empty the cistern, and inspect the flapper and the seat it rests on.
Step 4: Check the Fill Valve
If the float level is correct and the flapper is sealing properly but water is still running, the fill valve itself may be faulty. You can often hear a hissing sound from the valve even when the cistern is full. Fill valves are relatively inexpensive and available at Bunnings or Mitre 10.
Step 5: Check the Supply Line and Shut-off Valve
Make sure the shut-off valve behind the toilet is fully open and that the supply line isn't kinked or partially blocked, which can cause erratic filling behaviour.
What You Can Safely Fix Yourself
Replacing internal cistern components is not restricted plumbing work under New Zealand law, meaning a homeowner can legally do it themselves. Here's what's DIY-friendly:
- Replacing the flapper valve – Turn off the supply, flush, unhook the old flapper from the pegs on the overflow tube, and clip on a new one. Match the size carefully — most NZ toilets use a standard 50 mm or 75 mm flapper. Take the old one to the hardware store if unsure.
- Adjusting or replacing the fill valve – Replacement fill valves are universal-fit and come with instructions. The job involves turning off the supply, disconnecting the supply line, unscrewing the old valve, and fitting the new one.
- Adjusting the float – As described above, this is a simple adjustment requiring no tools.
- Cleaning mineral deposits – Soak the flapper seat and valve components in white vinegar to dissolve calcium build-up before replacing parts.
Safety note: Always turn off the water supply valve before working inside the cistern. If the shut-off valve is seized or won't close fully, do not force it — call a licensed plumber.
How to Prevent It Recurring
- Replace rubber components every 5–8 years as a matter of course — rubber degrades over time regardless of use.
- Use a toilet cistern tablet sparingly — some bleach-based tablets can accelerate rubber deterioration.
- Check your water pressure — excessively high mains pressure (above 500 kPa) can stress fill valves and cause premature failure. MBIE Building Performance provides guidance on acceptable pressure ranges under the NZ Building Code (G12).
- Inspect the cistern annually — a quick look inside takes 30 seconds and can catch problems early before they waste water.
- If you're on a metered water supply, consider fitting a dual-flush cistern to reduce consumption — Energywise has useful tips on water-efficient fixtures.
When to Call a Professional
While internal cistern repairs are DIY-friendly, there are situations where you should call a licensed plumber:
- The shut-off valve behind the toilet is seized or leaking — replacing or repairing isolating valves is restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006.
- There is water leaking from the base of the toilet or from the cistern-to-pan connection — this may involve the pan collar or cistern bolts and could indicate a more serious issue.
- The toilet is very old and parts are no longer available — a plumber can advise on replacement options.
- You've replaced the flapper and fill valve and the toilet still won't stop running — there may be a crack in the cistern or a problem with the pan itself.
- You're a tenant — report the issue to your landlord promptly, as a running toilet may be a breach of the Healthy Homes Standards. See Tenancy Services for your rights.
All plumbing work involving pipes, valves, and connections to the water supply must be carried out by a tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). You can find a qualified local plumber on tradedirectory.co.nz.
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