Installing an Under-Sink Water Filter

Most single-stage and multi-stage under-sink water filters are designed for DIY installation. The core skill is being comfortable turning off a water isolation valve and connecting a push-fit fitting. If that is fine, the install takes 30 to 90 minutes.

This guide covers the standard install. Reverse osmosis systems with drain saddles and storage tanks are more involved — for RO, follow the manufacturer's specific instructions or hire a plumber.

What you need

Tools:

• Adjustable spanner (shifter)
• Phillips-head screwdriver
• Bucket and old towel
• Drill with hole saw (only if drilling for the filtered tap — many sinks have a pre-cut hole)
• Spirit level (for mounting the filter housing if needed)

Fittings (most kits include these):

• T-piece adapter for the cold-water line, sized to your existing fitting (typically 1/2" BSP or 15mm push-fit)
• Push-fit tubing, usually 1/4" or 3/8"
• Dedicated filtered-water tap (separate from the kitchen mixer)
• Mounting bracket for the cartridge housing

Check the fittings before you start. Australian cold-water lines under sinks are usually 1/2" BSP threaded or 15mm push-fit; some kits ship with 3/8" or 22mm fittings, which need an adapter.

Step-by-step

1. Turn off the cold-water isolation valve under the sink. Open the kitchen tap to release pressure. Place the bucket and towel under the join you are about to break.
2. Disconnect the cold-water line from the kitchen mixer. Catch any water that comes out.
3. Fit the T-piece adapter. The T-piece sits between the isolation valve and the kitchen mixer's cold inlet. Tighten by hand, then a quarter-turn with the spanner. Do not over-tighten.
4. Reconnect the kitchen mixer to the new T-piece outlet. Cold supply to the mixer is now restored.
5. Mount the filter housing somewhere accessible inside the cabinet. The cartridge needs to be replaceable without disassembling the install — leave clearance below the housing. Use the included bracket or screw the housing directly to the cabinet wall.
6. Run the push-fit tubing from the T-piece tap to the housing inlet. Cut the tubing square with a sharp blade, push the end into the fitting until you feel the click, then tug gently to confirm it is seated.
7. Run the outlet tubing from the housing to where the dedicated filtered tap will sit. Same push-fit technique on the housing end.
8. Mount the filtered tap. If a pre-cut hole exists, feed the tap through and tighten the retaining nut underneath. If you need to drill, use a hole saw sized to the tap shank (typically 12mm or 16mm).
9. Connect the outlet tubing to the filtered tap inlet with the push-fit fitting on the bottom of the tap.
10. Open the cold-water isolation valve slowly. Run the kitchen mixer first to check there are no leaks at the T-piece. Then open the dedicated filtered tap and run the first three to five litres through to flush the cartridge — output will be cloudy initially as the carbon dust clears.

When to call a plumber

• Anything cutting into mains-pressure pipework rather than the existing isolation-valve setup.
• Drilling a stone or composite benchtop.
• RO systems with drain saddles (a saddle clamp on the sink waste line — NSW Health considers some drain-saddle installs to need a licensed plumber).
• Any whole-house system. Whole-house must be installed by a licensed plumber in NSW and most other Australian states.
• If you find rotten plywood, copper pinhole leaks, or anything else suggesting an existing issue under the sink — fix the existing problem first.

A leaky push-fit fitting is the most common DIY mistake. If you spot a drip in the first 24 hours after install, turn the water off, disconnect the fitting, check the o-ring is intact and the tubing is cut square, push back in until it clicks. If it still leaks, the fitting is damaged — replace it rather than trying to live with it.