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Rainwater Harvesting

So you want to install a rainwater tank?

 Diverting rainwater from your downpipe to a storage tank is a great way to conserve water, especially for gardening or outdoor cleaning use.

It can also increase your options in the event of a water supply emergency.

It can be relatively simple to install a kit that diverts the 'first flush' of rain from your roof and gutters, which contains most of the contaminants, away from your tank, feeds clean water into your tank, then directs overflow back into your stormwater system.

We encourage the installation of rainwater tanks, including for use in an emergency, such as an earthquake.

We’ve compiled a list of Frequently Asked Questions (see below) to provide guidance if you’re thinking about installing a rainwater tank.

Why should I install a rainwater tank?

Rainwater tanks offer multiple uses. They’re good for storing water for watering your garden and washing your car. And they’re also an excellent source of emergency water in a natural disaster – although you will need to treat the water if you want to drink it.

As we saw with the Canterbury earthquakes, a large earthquake can disrupt water supply for weeks or even months.

By diverting rainwater from your roof to a rainwater tank, you’ll have your own source of water, even when the water supply system isn’t working.  It will also mean you can continue to water your garden when water restrictions are in place, as long as you are using your own rainwater supply.

Can I drink the water from my rainwater tank?

For information on how to safely treat unfiltered water from your rainwater tank to use as an emergency drinking supply, visit the Nelson Marlborough District Health Board Website.

It is important to ensure that the water going into the rainwater tank is as clean as possible. We recommend installing leaf guards* and first-flush diverters+ on the downpipe leading to your rainwater tank. 

*A leaf guard is a screening device that is attached to your downpipe and prevents leaves and other debris from entering the rainwater collection system.

+First-flush diverters are designed to prevent the first part of the rainfall which picks up most of the dirt, debris and contaminants (such as (such as dust, pollen and bird droppings) from your roof, from entering the rainwater tank. 

It does this by diverting the initial first few millimetres of rainfall into a separate chamber. Once the roof is clean and the chamber is full, the rest of the water carries on to the rainwater tank.

How much water would I need for a natural disaster?

This depends on: (1) how many people there are in your household; and (2) how long it will take to restore the water supply network after a natural disaster.

Civil Defence recommends storing a minimum of three litres of water per person per day for seven days – but this is just drinking water for survival.

You will need to store more water for cooking and hygiene – we recommend having a minimum of 20L per person per day of emergency stored water for as long as the water supply system isn’t working.

If you have a family of four and the water supply is out for two months, you would need to have stored around 4,800L. A rainwater tank that is connected to your roof guttering system would get replenished every time it rains, so consider your local climate and rainfall whilst thinking about tank sizes.

What size tank would be suitable for household water storage

Bigger is better!

For maximum effectiveness, the tank must be connected to the roof guttering of your house so that it can be refilled by rainfall.

The amount of rain you can collect from your roof depends on the size of the roof and your local average rainfall – see table below for some local info:

The drier your climate, the bigger tank you need to make sure you have enough water to see you through an emergency period or a drought.

If you are looking for a simple system to store water simply for watering the garden, washing the car etc. then a 250 – 400 litre tank will usually be more than adequate for your needs, and easy to install yourself. You can of course add more tanks to increase your supply. Gardens use about 20% of a household’s water.

Laundry and toilet flushing uses up to 45% of a household’s water.

If you want to use rainwater for all your household needs all year round, including showering and drinking water, allow at least 300l per day per household, and make sure your tank has plenty of capacity for dry periods. Bathrooms and kitchens use about 35% of a household’s water.

What can a rainwater tank be made of?

A rainwater tank can be made of a variety of materials.  If the tank is going to be used for drinking water (even just in an emergency situation), the material that it is made from must comply with potable water requirements under AS/NZS 4020 'Products for use in contact with drinking water' and with AS/NZS 2070 'Plastics materials for food contact use'.

Types of tanks:

Polyethylene tanks: Commonly known as 'poly' tanks, these tanks come in many sizes and colours. They last a long time, are UV-resistant, cost less than metal tanks and, because of their lightweight construction, are easy to transport.

Metal tanks: Metal tanks are light and easy to transport, can be custom made and are usually corrugated or straight rolled. They can be made from a variety of metals including:

- Galvanised steel - zinc-coated Z600 steel (prone to rusting)

- Aquaplate or Colorbond - coloured polymer-coated steel (lasts longest)

- Zincalume - silver-coloured zinc/aluminium-coated steel (prone to rusting)

- Copper and stainless steel - used for specialised applications

Both metal and poly tanks are best for above ground use - they need to be reinforced if they are to be used below ground.

Concrete tanks: Concrete tanks can be built above or below ground. They're usually made on site and are durable and long lasting. However, they can sometimes crack – especially when they are below ground in clay soil. They're good for preventing algal growth (light can't penetrate) and they keep water cool.  Concrete tanks are generally only available in large sizes – over 9,000 litres

Fibreglass tanks: Fibreglass rainwater tanks are resistant to chemical corrosion and are suitable for both ground and stand installations. They are tolerant of extreme temperatures, come in a large range of colours and sizes and, because of their lightweight construction, are easy to transport. Fibreglass tanks can be more expensive than other varieties

Bladder tanks (or pillow tanks): Bladder tanks are made of flexible and durable PVC and are designed to fit under floors and decks.  They utilise previously wasted space rather than garden space.  The ground that the bladder tank sits on must be flat

Timber tanks: Timber tanks have timber exteriors (including a roof) with a plastic lining.  Smaller tanks can be erected by the homeowner.  They are available in a wide range of sizes

How much do tanks cost?

Rainwater tanks come in a variety of sizes and prices.

A 250L rain barrel typically costs around $200, and you’ll also need some hose pipe and fittings, and a diverter to send the water from your downpipe into the tank. Make sure the diverter has a leaf filter on it.

A 5,000L rainwater tank can cost up to $2,000. Installation costs (for example leaf guards, first flush diverters, earthworks, plumbing and pumps for indoor use) and transportation costs are extra. If you are installing a tank for water to be used as your potable supply, you’ll need to add in building consent and compliance costs.

Where can I buy a rainwater tank?

There are many retailers that sell rainwater tanks. Tanks less than 1,000 litres in size may be available from your local hardware store. Larger tanks can be sourced from a specialist manufacturer – google ‘domestic water tanks’ for suppliers in your area.  

Do I need planning or building consent?

For outdoor use - urban areas

If you're installing a typical (say 2,000 litre) tank to collect rainwater for outdoor use only, such as garden watering or emergency supply, then as a general rule you don't need a building consent.

Schedule One of the Building Act list situations where tanks do not require building consent: https://schedule-1-guidance/part-1-exempted-building-work-tanks

Please note however that other considerations may apply, such as the resource consent requirements for your area.

For example, if you were planning to put your tank on an elevated platform, you would still need to observe the height and boundary limits that apply in your area, and there are capacity limits relating to the height above ground as well. Call your local council if this applies to your situation

For indoor use

If you're planning to connect your rainwater tank to your toilet or washing machine, you will need a building consent.

This is to ensure pipe entry to the house is properly sealed, and that rainwater from your system can't enter the public water network. You will need a registered plumber to carry out this work for you.

And if you want to use it for drinking, you'll need to have the water treated or purified, and may need an annual inspection. Again, contact your local council's building consents team for details.

Does it need to be installed by a professional?

If you are connecting a rainwater tank to your toilet, laundry and or kitchen, the tank will need to be installed by a qualified tradesman and a building consent is required. Otherwise, most installations can be done by the homeowner – check with the supplier of your rainwater tank.

Does my rainwater tank need to be secured in case of an earthquake?

We recommend securing your rainwater tank so that in an earthquake, the contents are not lost – just when you would need it most! The rainwater tank needs to be secured in such a way that it does not cause damage to either your house or your neighbours’ properties.

Talk to your rainwater tank supplier for the best way to secure your tank.

Check out these links for more information:

Smarter Homes - collecting and using rainwater

Health Education - Water Collection Tanks and Safe Household Water

The Department of Building and Housing's schedule of exempt building work has details regarding tank construction. 

The Ministry of Health has a series of publications related to drinking water systems

Learnz has educational resources about water use available here. 

Download Council's handy guide to rainwater harvesting