Rainwater tanks and greywater for lawns: what works and what's allowed

When water is short, the roof and the laundry start to look like free sources of lawn water. Both can help. A rainwater tank is the simpler and safer option, and can supply a lawn for part of the year. Greywater can supply a lawn too, but it carries germs and chemicals, so it comes with rules designed to protect your family and your soil.

This guide covers what a rainwater tank can realistically do, what an installation needs, and when and how greywater can be used on a lawn. Rules differ by state, council and water authority, so treat this as a guide to what to check, not a substitute for your local rules.

Rainwater first

NSW Health’s greywater policy makes a useful point: water conservation and rainwater harvesting for garden watering have less environmental and public health impact than greywater reuse, and are the first options to consider. Before you plumb greywater to the lawn, make sure you’re watering efficiently. The how much water does a lawn need guide covers that.

How much can a tank supply?

The sums are simple, because one millimetre of water over one square metre is one litre.

• Collection: every millimetre of rain on each square metre of roof that drains to the tank gives up to about one litre. A 100 m² roof in 10 mm of rain collects up to about 1,000 litres. Some is lost to the first-flush device and to gutters overflowing in heavy rain.
• Lawn use: applying 10 mm to a 100 m² lawn takes about 1,000 litres.

So a 5,000-litre tank holds about five deep waterings of a 100 m² lawn. That’s useful in a dry spell, but in a long, hot summer with little rain, a tank of that size won’t carry a lawn of that size on its own.

Making a tank go further

• Water efficiently first: deeply and less often, early in the morning, after a catch-cup test.
• Prioritise the lawn areas that matter most, and let low-use areas brown off.
• Use a wetting agent on water-repellent soil so tank water soaks in rather than running off.
• Mow higher in summer.
• Use a mains backup where it’s allowed and properly plumbed, so you’re not left without water.

Use the Lawn Area Calculator for your lawn, and measure the roof area that drains to the tank, to do your own sums.

Choosing a size

A bigger tank stores more of a heavy downpour, but it only helps if your roof can fill it. Work out how much your roof collects in a typical month of rain, how much your lawn and garden use in a dry month, and how much space you have. Your council may also limit tank size, height and position near boundaries.

Installing a rainwater tank

Requirements differ by state and council. Sydney Water’s guidance for NSW gives a good picture of what to check:

• Approval: tanks up to 10,000 litres don’t normally need council approval in NSW if they meet the NSW Housing Code conditions. Larger tanks, or tanks in other states, may need approval. Ask your council.
• A licensed plumber: Sydney Water recommends a licensed plumber experienced with rainwater tanks. In NSW, the plumber must book a NSW Fair Trading inspection if the tank connects to a toilet, washing machine or hot water system, or is topped up by or switched to drinking water when it’s empty.
• Backflow prevention: check your backflow prevention requirements. You may need a device that stops tank water flowing back into the drinking water supply.
• Mosquito-proofing: the tank inlet and overflow must be mosquito-proof, with no other way in.
• A first-flush device, which diverts the first, dirtiest water from the roof so cleaner water goes into the tank.
• An overflow to the stormwater system.

Pumps

Most lawn irrigation needs a pump to deliver enough pressure. Sydney Water notes that external pumps are generally more durable, while submersible pumps are quieter but can need specialist repairs.

Match the pump to your sprinklers. Every sprinkler needs a certain flow and pressure to work as designed. If you’re putting in an irrigation system, design it around the pump’s flow and pressure, not the mains. Drip and sub-surface lines work at lower pressures than many spray heads and rotors. See the DIY irrigation design guide.

Looking after a tank

• Keep gutters clean so leaves don’t block the inlet.
• Check screens and the first-flush device regularly, and empty the first-flush chamber as its maker directs.
• Check mosquito screens are intact.
• Keep a filter between the tank and the sprinklers, and clean it.

Greywater: what it is

Greywater is household wastewater that doesn’t come from a toilet: water from showers, baths, hand basins, washing machines, laundry tubs, kitchen sinks and dishwashers. It’s different from blackwater, which comes from toilets.

It isn’t clean. NSW Health describes greywater as containing micro-organisms, some of which can cause disease, as well as dissolved salts, detergents, oils, lint and hair. Some sources are worse than others:

Greywater: the three ways to use it

Health departments generally recognise three approaches. What’s allowed, and what needs approval, depends on where you live.

Bucketing

Carrying greywater out in a bucket. Western Australia’s code of practice says bucketing needs no council approval there if done as the code describes, but its list of rules includes not applying greywater where children, pets or people with low immunity can readily reach it, not using it in rain, not letting it pool, and not using it on water-repellent soil. For a family lawn, that rules out a lot.

Greywater diversion devices

A diversion device sends greywater straight from the house to the garden, without storing or treating it. Because the water is untreated:

• it must go through sub-surface irrigation, buried below the surface (at least 10 cm in WA) so people and pets don’t touch it;
• it must have a switch or valve to send water back to the sewer when the garden doesn’t need it, such as in wet weather;
• it must not be stored untreated. WA allows a surge tank to hold it for less than 24 hours.

NSW Health specifically warns against diverting washing machine water straight onto a lawn where children and pets may be exposed. For a lawn, a diversion device means sub-surface drip lines laid under the turf, ideally before the lawn is laid.

Greywater treatment systems

A treatment system collects, treats and usually disinfects greywater, so it can be stored and used more widely. In WA, approved systems that treat and disinfect to the required standard can be used for surface irrigation. These systems cost more, need regular maintenance, and must be approved for use in your state.

Approvals

• In Western Australia, local government approves greywater systems for single houses.
• In New South Wales, greywater systems in sewered areas are covered by NSW Health policy and local council approvals.
• In the Northern Territory, check the NT Government’s requirements for reusing greywater at home.
• Elsewhere, ask your council and water authority.

Plumbing greywater from the house is plumbing work, and in most places that means a licensed plumber.

Looking after the soil

Greywater adds salts, especially sodium, which can damage soil structure over time. To reduce the risk:

• Choose detergents low in salt, phosphorus and boron. WA’s code notes that liquid laundry detergents generally contain the least salt and ordinary powders the most.
• Spread greywater around, so it doesn’t build up in one place.
• Switch back to the sewer when the soil is wet, and from time to time to let the soil recover.
• Watch the lawn and soil for yellowing, poor growth or a crust on the surface, and stop if you see them.

When not to use greywater

• When someone in the house is sick, especially with gastro.
• After washing nappies, soiled clothing, or rags used for paint, oil or chemicals.
• When cleaning products, hair dyes or other chemicals have gone down the drain.
• In wet weather, or when the lawn doesn’t need water.
• On vegetables eaten raw.

Water rules for tanks, bores and greywater

Some water authorities apply their watering rules only to mains water. South East Water and Yarra Valley Water in Melbourne, for example, say their permanent water saving rules apply only to drinking water. Others apply rules to bores or have their own conditions. Check your water authority’s rules before you rely on any alternative source. The Bureau of Meteorology’s national water restrictions search is a good place to start, and the watering under restrictions guide links each capital city’s official page.

Common mistakes

• Expecting a small tank to water a large lawn all summer.
• Connecting a tank to the mains without a licensed plumber and the right backflow protection.
• Leaving tanks open to mosquitoes.
• Running a hose from the washing machine onto the lawn.
• Storing untreated greywater in a tank or bin.
• Using greywater in wet weather or on soggy soil.

Log it

Log tank and greywater watering in your Lawn Journal alongside mains watering, noting the source. Over a season, you’ll see how far your tank goes, and how much mains water you’ve saved.