Greywater Diverter Install: Off-Grid Aussie Guide

Sarah Chen · environmental engineer · water, waste and everything that flows downhill

I installed my first greywater diverter on a 1000-litre septic tank in the Adelaide Hills in January, when the tank was backing up every third washing cycle. The kit was a Davey diverter, $385 from a plumbing supplier in Mount Barker, and the tool list was short: a cordless drill with a 25 mm spade bit, a hacksaw, a tube of silicone sealant, and a roll of 50 mm PVC pipe left over from a rainwater tank job.

First Flush Diversion To Garden

The instructions said to cut into the washing machine outlet and divert the first flush—the lint-heavy, soapy slug—to the garden, not the septic. What they didn’t say was that in summer, if your garden hose runs 30 metres to the lemon trees, you need a 19 mm non

Install Diverter Above Overflow

I ran the diverter outlet into the top of a 22,700-litre poly rainwater tank near our workshop at Boorowa. The tank had an existing brass 25 mm overflow fitting, standard on a Rhino tank of that vintage, positioned 150 mm below the top rim. I used a common Ozito 18 V drill and a 32 mm holesaw to punch a new inlet just above that overflow. Hose was 25 mm rural poly from the diverter, a Waterwork GDD-300 unit that cost $197 at the time, into a brass hose barb and silicone sealant. It looked tidy.

Tank Drained Backwards Through Diverter

First decent rain, the diverter kicked on, greywater filled the top of the tank, and within a day the water level dropped until the pump lost prime. Cause was simple: the new inlet sat higher than the overflow. When the pump drew down, air entered the overflow pipe, broke the siphon, and the tank drained backwards through the diverter’s unsweated check valve under gravity. Three thousand litres of captured rainwater ran out across the paddock—enough to supply our two toilets and washing machine for close to seven weeks.

Fix Cost And Materials

The fix cost me a 25 mm nylon elbow, a metre of poly, two stainless worm-drive clamps ($14 total at the local CRT) and an afternoon moving the inlet 80 mm below the overflow. The tank’s been stable for two years since.

The laundry trough drains through a 40 mm PVC pipe that runs down the wall and elbows straight into the septic inlet. Cutting into that line costs less than a six-pack of Melbourne Bitter. A manual three-way diverter valve — usually black ABS, with a blue handle marked “Tank / Sewer” — sits at the junction. These valves sell for $30 to $50 at any plumbing supplier. The T-piece and two 40 mm compression unions add another $12. Total materials land under $70. No electric pumps, no float switches, just a ball valve you flick by hand.

First time I fitted one, I backed the hacksaw through the downpipe and a cup of stale rinse water sloshed onto my boots. The pipe stub needed a clean 45-degree chamfer with a half-round file so the compression ring wouldn’t bite sideways. Slather the rubber with silicone grease, not petroleum jelly, or the seal swells and leaks within a month. Tighten the nuts finger-firm plus a quarter turn with a strap wrench — overtightening splits the collar. Run a 50 mm gravity line out through the brick veneer wall with an 8 mm-per-metre fall.

Less fall and lint settles; more and the water outruns the solids, leaving sludge behind. A 90 mm core drill hole sealed with UV-stabilised neutral-cure silicone keeps the ants and rain out. Outside, daylight the pipe into a surge tank or a purple-handled distribution hose — purple is the Australian standard colour for non-potable water lines. The hose ID matters: 19 mm soaker hose kinks if you look at it sideways; 13 mm dripline works but clogs fast on laundry lint without a 100-micron inline filter.

The filter came from a pump shop, an opaque canister with a washable stainless mesh that catches hair, fluff, and the occasional LEGO brick a kid left in a pocket. Clean it once a fortnight or the backpressure sends sullage up the shower drain. That smell, when it happens, is unmistakable — hot soap and dying bacteria, drifting through the bathroom grating at 7 a.m. Not the fresh start you wanted. Shower diversion runs the same principle but the pipe trap must stay wet. Dry traps let septic gas backflow into the room.

If the shower hasn’t been used for three days, pour a litre of water down the grate before switching the diverter. The measurement isn't fussy; any water that fills the P-trap works, but 1 L covers a standard 50 mm trap with some to spare. A sink diverter is the shortest run because the basin is already high. From the 32 mm trap outlet I adapt up to 40 mm immediately — smaller diameter traps catch solids faster. A rubber reducing coupling with two stainless steel clamps costs $6 and takes the guesswork out of gluing different pipe sizes.

Slope it, strap it to the stud with a clip every metre, and terminate in a raised goose-neck so the tank side never back-siphons. Cost-wise, from laundry to distribution hose, $90 to $140 covers everything if you do the digging and assembly yourself. Time is a long morning — three hours if the ground is soft, five if you hit clay and the shovel rings like a bell.

No permit needed for sub-200-square-metre irrigation area in most states, but check the local council’s greywater fact sheet because the rules read like they were written by a committee that never buried a pipe in its life. The only moving part is the diverter handle. Spray it with food-grade silicone lube once a season. If it seizes, hot water from the kettle over the valve body usually frees the ball. If not, the whole unit unbolts with four screws and a new cartridge swaps in ten minutes. Keep the original box with the spare O-rings taped inside.

You will lose them otherwise.

Install A Basic Laundry To Garden Diverter

Stop wasting water. Install a greywater diverter for laundry, shower, and sink. No fluff. Just the facts. A basic laundry-to-garden diverter is the quickest win for most off-grid setups. A manual three-way valve — Nylex, Holman, or the no-name poly version from the irrigation aisle — sits inline on the washing machine outlet hose. The valve costs between $60 and $90. Flick it left and the wash water runs to the septic or sewer. Flick it right and the same water heads out a 25 mm poly line to the garden. No pump. No mains connection.

Designing Gravity Drainage Lines

Gravity does the work, provided the garden end is lower than the machine’s outlet. A washing machine pump can push suds about 30 metres horizontally across flat ground before flow turns to a dribble. Slope the distribution line at 1:100 (a 10 cm drop over 10 metres) and it self-drains enough to stop mosquitos breeding inside the pipe. The first time I cut into a laundry drain in a 1970s weatherboard farmhouse at Murrindindi, the accumulated lint and soap sludge looked like a family of mice had died inside the 40 mm PVC. Keep a bucket under the cut.

Filtering Greywater Debris

Wet wipes, hair ties, and a surprising number of Lego pieces had marched past the machine outlet over the years. A coarse in-line lint filter screwed onto the hose before the diverter catches nearly all of that before it hits the garden drippers. A clear-body filter like the Davey Rainbank inline strainer (around $35) lets you see the grey

What You Need

One diverter valve per source. That single valve mounts on the waste pipe of one fixture — typically the laundry trough or a ground‑floor shower — before it joins the main drain. It is not a treatment system. It is a manual three‑way ball valve, usually 50 mm or 40 mm DWV PVC, that switches flow between greywater (garden) and blackwater (septic). Flick the handle one way and the wash water runs out through a subsurface irrigation line to the citrus row. Flick it back and everything heads to the septic tank as normal.

Selecting The Right Ball Valve

In a typical off‑grid shed‑and‑paddock setup, you buy a plain 50 mm PVC three‑way ball valve from an irrigation or rural plumbing supplier for $60 to $120. A lockable‑handle version costs a bit more but stops a curious child or a visiting grey nomad from accidentally watering the septic inlet. You also need a short length of 50 mm DWV pipe, a T‑junction if you are picking up two fixtures, solvent cement, a hacksaw, and a deburring tool. The whole thing rarely cracks $200 per source.

Install The Garden Leg

The install takes a morning under the house or behind the laundry wall, provided you can reach the waste pipe before it drops into the inspection opening. The valve spigots slip straight into standard PVC sockets, so you cut out a section of the vertical drop, prime and glue the valve in place, and run the garden leg through a hole in the weatherboard. Slope that garden line at 1:50 minimum so solids and lint keep moving instead of turning into a sour, mid‑pipe pudding.

Divert Greywater To Lemon Trees

When the machine finishes its rinse cycle, you step out, turn the handle, and the soapy greywater soaks the roots of the old lemon tree instead of adding hydraulic load to the septic drainfield. The tree gets a feed of nitrogen and phosphorus; the septic gets a breather. Legal in all states for garden use. Every state rulebook permits diverting untreated greywater from a single fixture onto your own garden, as long as the water is applied below the surface and not stored for more than 24 hours.

Check your council for local setback distances from boundaries, bores, and waterways — but usually it’s fine.

Laundry Diverter Install

Diagram

Steps

1. Unplug washing machine. Disconnect drain hose from main pipe.
2. Cut a section out of the 50 mm PVC drain line below the laundry trough or washing‑machine standpipe, leaving enough room to solvent-weld in a 50 mm PVC T‑fitting with the branch pointing toward the garden. On that branch, thread a 50 mm PVC ball valve — a manual three-way diverter — so you can swing flow to the paddock or back to the septic. Bunnings sells a perfectly functional grey 50 mm ball valve for around $18, and a 50 x 50 x 50 mm T‑piece for $4.50. Wrap all threaded joins with three turns of pink Teflon tape, tighten with a large shifting spanner until snug, not white‑knuckled. If the washing‑machine hose is a 25 mm flexible rubber tail, slip a barbed 25 mm to 19 mm garden‑hose adapter onto the outlet side of the ball valve and secure it with a stainless‑steel worm‑drive clamp — total cost under six dollars. Any arm running to the garden needs a minimum fall of 1:50 (20 mm drop per metre) or greywater sits, stinks, and breeds a mosquito motel within a week. I welded a 6‑metre run of 50 mm DWV pipe from the valve straight to an old apple‑crate grease trap, and that shallow slope ate every scrap of lint without a blockage for three years. When the valve handle is parallel to the pipe, water goes to the drain; crank it 90 degrees and everything heads outdoors. Fixed handles work better than the plastic lever types that go brittle after a summer — my neighbours learnt that after the lever snapped mid‑laundry load and flooded the shed with soapy water at 7 a.m.
3. Run greywater pipe (50mm PVC) to garden. Run blackwater pipe to septic.
4. Test: Set valve to greywater. Run a cycle. Check for leaks. Switch to blackwater. Repeat.

Tip: Use a 50mm PVC pipe. Bunnings sells it for $1.50/m. Don't use 32mm – it'll clog.

Shower & Sink Diverter

Diagram

Steps

1. For shower: Cut drain pipe below P-trap. Install diverter valve.
2. For sink: Remove P-trap. Install diverter valve in drain line.
3. Connect pipes as above. Use 40mm PVC for sinks (smaller flow).
4. Test both settings. No leaks = done.

Tip: Shower water is high in soap. Use a mesh filter at the garden end ($5 at Bunnings). No bleach – it kills plants.

Australian Suppliers & Pricing

Don't overpay. These are the real deals:

• Aqua-Valve Diverter (best for laundry): $120-$140. Plumbing Warehouse or Bunnings (online). Bunnings has a similar one for $105.
• Greywater Pipe Kit (diverter + 5m PVC): $220. Green Source (no need – DIY is cheaper).
• PVC Pipe & Fittings: $35 total. Bunnings (50mm PVC pipe, 2x 90° bends, 1x T-fitting).

Total cost: $150-$175. No plumber needed. Takes 2 hours. If you're not doing it yourself, you're wasting money.

Final Word

Greywater diverter = water saved = money saved. Install it. Don't wait. Your garden will thank you. Your wallet will thank you. Done.