Harvesting rainwater

Climate change, population growth and other factors mean water is an increasingly precious resource. Installing a rainwater tank can be an effective way to help manage our water resources better.

How it works

Rainwater tanks collect water from your roof when it rains, by connecting to a downpipe. They’re common in rural areas with no towns water supply but as awareness about the need to conserve water grows, they’re becoming increasingly common in homes that are also connected to mains water in towns.

Rainwater tanks help ease the demand on the town supply. You can use harvested rainwater for things like watering the garden or washing the car. You can also use it for flushing toilets or washing clothes, by connecting your tank to the plumbing in your house. 

We’ve focused here on ‘supplementary’ rainwater tanks for homes that are also connected to the mains supply. If you’re thinking about using tanks as your main source of water or don’t have access to a mains supply, we recommend you get expert advice.

What size do you need?

Rainwater tanks to supplement your mains supply come in a range of sizes, from 200 litres to upwards of 5,000 litres if you’re planning on indoor use as well. How big a tank you need depends on factors like what you’re using it for, how much rain you get on average, and the size of your roof. Tank providers can help you get the right size for your needs.

Water collected from your roof can contain things like dirt and debris, bird and animal droppings, ash and other residues, so it’s generally not suitable for drinking. In an emergency, though, you can drink it if you boil it first or add chlorine – provided your roof doesn’t contain lead-based paint or uncovered lead flashings etc.

Homeowners can install smaller tanks themselves e.g. for the garden and outdoor use however check with the provider to ensure they are suitable for your needs (Source: Wellington City Council). If you’re planning to connect a tank to your indoor plumbing, you’ll need a qualified plumber to install it. 

While you typically won’t need a building consent if you’re installing a tank for outdoor use, you will if you’re connecting it to your home. Depending on your particular situation there may be other instances where a building or resource consent is required – check with your local council first.

Smaller tanks for outside use typically rely on gravity and have a tap. Raising the tank onto a stand provides easier access. You’ll also need a pump for indoor use. 

How much it costs

From around $200 for a basic rain barrel to around $3,000 for a 3,000 - 5,000 litre tank, depending on the design and what it’s made of.

You may also need to factor in costs for installation and/or consents, if necessary (Source: Auckland Council).

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The key is keeping contaminants out of the water as much as possible. You’ll need to regularly clean your spouting (at least the part that delivers water to the downpipe your tank is connected to), keep leaf guards, filters etc. free of dirt and debris, and clear any overhanging vegetation.

You’ll also need to inspect your tank for sediment accumulating on the bottom every two to three years on average and clean it out before it reaches the tap or outlet.

If your tank is connected to your home, you’ll need regular inspections to ensure it’s kept separate from the rest of the plumbing.

Benefits — for you and the planet

Water conservation

Water resources are under increasing pressure, especially with the impact of climate change. Collecting rainwater helps by reducing demand for water from other sources.

Reduced runoff

Collecting water from your roof means less of it ends up in storm water systems and ultimately in waterways, where it can reduce water quality and lead to flooding, erosion and contamination of our beaches from sewage overflow.

Good for your garden

Rainwater has fewer of the chemicals and treatments often found in mains water. It also contains nitrogen in the form of nitrates, that can be absorbed by plants to support healthy growth.

Is it right for you? 

Rainwater tanks can be a cost-effective way to harvest a renewable supply of water. If you just want a small tank as part of your emergency preparedness planning, or an environmentally friendly way to water your garden, they’re simple to install and relatively inexpensive. 

If you want to use rainwater in your home as well, you’ll need a bigger tank and you’ll need a plumber to install it. You may also need building and/or resource consent – so you’ll need to weigh up the extra costs and complexity against the benefits for your situation.

Other things to think about

To keep leaves, dirt and debris out of your water tank consider installing leaf and gutter guards. You may also want to consider a ‘first flush diverter’. When it rains, the initial flow of water is more likely to contain dirt and contaminants from your roof. A first flush diverter helps prevent this from entering your tank. 

You’ll also need to make sure any overflow from your tank when it’s full, is directed back into the storm water system so it doesn’t cause damage – either back into the downpipe via a diverter or directly into a storm water drain. Providers and installers will be able to advise you on the best approach.

How to get a rainwater tank for your home


HomeFit self-assessment is a straightforward way to check if a home is warm, dry, efficient and safe. You can use it to check a home you’re looking to buy or rent, or if you want to know how you can improve your current home. It starts with a simple online check.

If you would prefer to speak to someone you can contact an independent HomeFit assessor who can visit your home to provide a detailed assessment and list of priorities. 

HomeFit was developed by the New Zealand Green Building Council and is proudly supported by ANZ.

Important information

The material is information only and you should seek professional advice about your circumstances. While we’ve taken care to ensure the information is reliable, we don’t warrant its accuracy, completeness, or suitability for your intended use. To the extent the law allows, we don’t accept any responsibility or liability arising from your use or reliance on this information.