Installing an under-sink water filter is one of those satisfying weekend DIY jobs that's well within reach for a reasonably handy homeowner — provided you stay within the rules. In New Zealand, some plumbing work is legally restricted and must be carried out by a licensed tradesperson, but connecting a simple filter to an existing cold water supply line generally falls outside that restriction. Here's what you need to know before you reach for the spanner.
What the Law Says About DIY Plumbing in NZ
Under the Plumbers, Gasfitters and Drainlayers Act 2006, certain plumbing work is restricted work that only a licensed or certifying plumber may carry out. This includes work on sanitary plumbing, drainage, and connections to the mains supply pipework itself. However, connecting a filter unit to an existing, already-installed cold water tap or supply valve — without altering the pipework — is generally considered minor maintenance work that a homeowner can do themselves.
If you're in any doubt about whether your specific situation counts as restricted work, contact the Plumbers Gasfitters & Drainlayers Board or ask a licensed plumber before you start.
Types of Under-Sink Filters
Understanding what you're installing helps you gauge the complexity:
- In-line filters – Fitted to the existing cold water flexi-hose between the shut-off valve and the tap. Minimal plumbing involved; usually the most DIY-friendly option.
- Single-stage or multi-stage canister filters – Require a dedicated feed line from the cold supply valve and a separate filter tap drilled through the sink or benchtop.
- Reverse osmosis (RO) systems – More complex; typically include a storage tank, multiple filter stages, a dedicated tap, and a drain connection. The drain connection may constitute restricted work — check with a licensed plumber.
What You'll Need
- The filter system and all manufacturer-supplied fittings
- Adjustable spanner and/or basin wrench
- PTFE (Teflon) tape
- Bucket and towels (for residual water)
- A drill with appropriate bit if fitting a dedicated filter tap
- Silicone sealant (for tap hole if required)
You'll find filter systems and fittings at Bunnings or Mitre 10. Always buy a reputable brand and check the product is rated for drinking water use.
Step-by-Step: Basic In-Line or Single-Stage Filter
1. Turn Off the Water Supply
Locate the cold water shut-off valve under the sink (usually a small quarter-turn or gate valve on the cold flexi-hose). Turn it off, then open the cold tap to release pressure and drain residual water into your bucket.
2. Disconnect the Cold Flexi-Hose
Unscrew the flexi-hose from the shut-off valve. Have your towel ready — a little water will spill.
3. Connect the Filter Unit
Follow the manufacturer's instructions exactly. Use PTFE tape on all threaded connections (wrap clockwise, 3–5 turns). Hand-tighten first, then snug up with a spanner — don't overtighten plastic fittings.
4. Fit a Dedicated Tap (if required)
If your filter needs its own tap, you'll need to drill a hole in the sink or benchtop. Many sinks have a pre-punched blanking plug for this purpose. Use the correct drill bit for your sink material (stainless steel, stone, or composite) and seal around the tap base with silicone.
5. Check for Leaks
Slowly turn the supply back on. Watch every connection carefully for at least two minutes. Tighten any weeping joints slightly — if a joint continues to leak, turn off the supply and re-do it with fresh PTFE tape.
6. Flush the System
Run water through the new filter for the time specified by the manufacturer (often 5–10 minutes) to clear any carbon fines or manufacturing residue before drinking.
NZ Building Code Considerations
The NZ Building Code clause G12 (Water Supplies) requires that drinking water systems don't create a health risk or contaminate the supply. Always use filter cartridges and fittings rated for potable (drinking) water. Check that your filter system meets any relevant standards — look for NSF/ANSI 42 or 58 certification on the packaging.
If your home is on a private water supply (tank or bore water), the BRANZ website has useful guidance on water quality and treatment options suited to NZ conditions.
Maintenance: Don't Forget the Cartridges
A filter is only as good as its maintenance. Most cartridges need replacing every 6–12 months, depending on your water quality and usage. Set a reminder and keep a spare cartridge on hand. Neglecting replacement can actually make water quality worse as bacteria can colonise a spent filter.
When to Call a Professional
Call a licensed plumber if:
- You need to alter or extend existing pipework to reach the filter location
- You're installing a reverse osmosis system with a drain connection
- You're unsure whether your shut-off valve is working correctly or if it's corroded
- There's any sign of existing leaks or damaged pipework under the sink
- You're on a continuous flow hot water system and want filtered hot water (this involves more complex work)
- You simply aren't confident — a small leak left undetected under a sink can cause significant damage
A qualified local plumber can have a basic filter installed quickly and give you peace of mind. You can find licensed plumbers in your area on tradedirectory.co.nz.
Safety Note
Always confirm the water supply is fully off before disconnecting any fittings. Never work on pressurised lines. If you smell anything unusual or notice discoloured water after installation, stop using the filter and contact a plumber.