Testing a drain pipe for leaks using a water test — sometimes called a hydrostatic test or water pressure test — is a reliable, low-tech method used by drainlayers across New Zealand. It's commonly required before a trench is backfilled on new or repaired drainage work, and it can also help you diagnose suspected leaks in existing systems. Here's how it works, what the rules say, and where the limits of DIY sit.
What Is a Water Test and Why Does It Matter?
A water test involves filling a section of drain pipe with water, holding it at a set head (height) of water pressure, and checking whether the level drops over a set period. A drop in level indicates water is escaping — through a cracked pipe, a poorly made joint, or a failed seal.
Under the NZ Building Code, drainage systems must be watertight. The relevant Building Code clause is G13 (Foul Water), which sets out requirements for drainage to be free from leakage. NZS 4404 relates to land development and subdivision infrastructure and may also be relevant depending on the scope of work — confirm applicable standards with your council or licensed drainlayer. A water test is one of the accepted methods to demonstrate compliance.
What You'll Need
- Drain test plugs (inflatable or mechanical) — sized to fit your pipe diameter (typically 100 mm for residential)
- A length of clear plastic tube or a standpipe to act as a gauge
- A garden hose or bucket to fill the pipe
- A measuring tape and a timer
- A notebook to record readings
Test plugs are available from trade suppliers and some hardware stores. Check Placemakers or Mitre 10 for availability, or hire them from a tool hire shop.
Step-by-Step: How to Perform the Water Test
1. Isolate the Section to Be Tested
Insert drain test plugs at both ends of the pipe section you want to test. Make sure all branch connections within that section are also plugged. The section should typically be no longer than what can be practically managed — your drainlayer or council inspector may specify the length.
2. Set Up Your Standpipe
Attach a vertical standpipe or clear tube to the upstream plug's fill port. This lets you fill the pipe and monitor the water level. The top of the standpipe sets your test head — the standard test head for residential drains is typically 1.2 metres above the top of the pipe at the highest point of the test section. Always confirm the required head with your local council or the inspecting drainlayer, as requirements can vary.
3. Fill the Pipe Slowly
Fill the pipe with water through the standpipe until the water reaches the required test head. Fill slowly to allow air to escape. Once full, allow the water to stabilise for at least 15 minutes — this lets the pipe absorb any initial moisture (especially important with concrete or clay pipes) and allows trapped air to escape.
4. Record the Starting Level and Begin Timing
Once the level has stabilised, mark or record the exact water level in the standpipe. Start your timer. The standard test duration is typically 30 minutes, but confirm this with your council or inspector.
5. Check for Drop
At the end of the test period, measure any drop in the water level. A commonly referenced acceptable leakage allowance for PVC pipes is very small — often expressed as a maximum drop in millimetres over the test period, depending on pipe diameter and section length. Check with your local council or a licensed drainlayer for the exact pass/fail criteria, as these can differ between councils and depend on the pipe material.
| Pipe Material | Notes |
|---|---|
| PVC (uPVC) | Very low acceptable leakage; joints must be properly solvent-welded or rubber-ring jointed |
| Concrete or clay (older homes) | Slightly higher allowance due to pipe absorption, but significant drops still indicate a problem |
| Cast iron | Rare in modern NZ residential; check with a drainlayer |
6. Locate the Leak (If the Test Fails)
If the level drops more than the allowable amount, you'll need to find the leak. Walk the trench line (if open) looking for wet soil or seepage. For buried pipes, a CCTV drain inspection by a licensed drainlayer is the most reliable next step.
Important Safety Notes
- Never enter an unsupported trench without proper shoring — check WorkSafe NZ guidance for the specific depth thresholds and requirements that apply.
- Keep children and pets away from open trenches.
- Ensure the test plugs are rated for the pressure you're applying and are properly seated before filling.
What Homeowners Can and Cannot Do
In New Zealand, drainlaying is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This means that installing, altering, or repairing drain pipes must be carried out by (or under the supervision of) a licensed drainlayer registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
As a homeowner, you may be able to carry out a water test on your own existing drainage for diagnostic purposes — but any repair work that follows almost certainly requires a licensed drainlayer. If the work requires a building consent, the drainlayer must also ensure the work is inspected and signed off. Some minor drainage work may fall under Schedule 1 exemptions, but always check with your local council before starting.
When to Call a Professional
Call a licensed drainlayer if:
- Your water test fails and you need to locate or repair the leak
- You're installing new drainage or altering an existing system
- You need a producer statement or inspection sign-off for building consent purposes
- You're unsure about the correct test head, duration, or pass/fail criteria for your council
- The drain is deep, in a confined space, or near other services
You can find qualified, local drainlayers on tradedirectory.co.nz.
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