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Rectangle water tank in the backyard near wooden fence
Collecting rainwater in water storage tanks and using it wisely can help see you and your garden safely through summer.

Liquid gold

Summers are predicted to get hotter and drier. And with water restrictions a possibility in parts of New Zealand this season, capturing and making good use of water to keep your garden looking its best throughout the season makes great sense. Collecting rainfall in a water tank is a smart first step in urban areas (and usually essential for regional zones), maximising every drop as well as helping you save money on your water bills. The benefits don’t end at your hip pocket, either – water tanks can help reduce run-off, which in turn protects stormwater drains and waterways.

Water service professional Julian Fyfe says, while using rainwater for drinking is discouraged, a rainwater tank will more than likely allow you to replace all the mains water you use flushing the toilet and in the laundry. And tank water may just save your garden when the heat is on.

Which water tank is best?

When choosing a new water storage tank, match supply to your demand, says Julian. “There’s no point getting a 20,000 litre tank if you’re not going to use all that water,” he says. “Think about your climate (the wetter it is, the less you need to store), what you’re planning to use it for, and the roof area you’ll be collecting from.” 

Figuring out the best-size tank for your property requires a bit of maths. Horticulture and sustainability specialist Adam Woodhams explains, “One millimetre of rain falling on one square metre of roof is one litre of water. You will never collect 100 per cent of fallen rain due to splashing and overrun, but knowing this figure is useful for doing a rough calculation of how much you have the potential to capture.”

Adam then follows with this equation to help choose the right size of water tank. “The maths is simple: annual rainfall for your area, multiplied by roof area to be connected, equals potential annual maximum,” he says. “For example: 1000mm x 50sqm = 50,000L. This means that, even with regular use, a 5,000 or 10,000L tank would provide consistent supply as it would likely be regularly toppedup.”

To get bang for your buck, Julian suggests you connect the tank to your toilet and laundry. “That way, you have a constant drawdown of the water – you don’t want it just sitting there,” he says. “But if you’re also planning to use it for the garden and you’re in a drier area, then the bigger the tank the better.” 

The ideal tank material depends on its use and location. Plastic water tanks come in a variety of shapes and sizes, but Julian Fyfe says if you’re looking for a big water tank, steel tanks are more suitable for storing more than 10,000 litres. Underground tanks are usually concrete.

Where to put your garden water tank

Slimline designs and colours can make new rainwater storage tanks a landscaping asset. However, Julian advises checking with your council for planning controls, such as the proximity to the street. Other factors to consider include not placing it under trees and sealing it to prevent animals getting in. “Lower temperatures help inhibit biological growth in the water, so tanks are better placed in the shade,” says Julian.

Consider the weight, too: 1000 litres of water weighs one tonne. You need to ensure the tank has a level, stable and strong base, such as concrete.

Water tank in the backyard near wooden fence

Useful water tank extras

Once you have decided on your tank, a first flush diverter is an important addition – this sends the ‘first flush’ of rainwater away from your tank, to help prevent debris washing in. You may also require a pump, says Julian. “If you’ll be watering your garden with a watering can, the pressure will be okay, but you will need a pump if you’re connecting the tank to indoors,” he says.  

Julian recommends a pressure vessel (a cylinder that sits over the head of the pump), so when someone turns the tap, the accumulated pressure pushes the water through the tank, so the pump doesn’t have to constantly turn on and off. “This protects the pump and your pipes by reducing ‘water hammer’, and stops your pipes potentially leaking and breaking,” he says.

Rainwater is a gift for the garden

Water tanks have other gardening perks, too. Plants and soil seem to respond better to natural water than town water, and Adam says there are a few reasons why. The pH of rainwater is likely a better balance – though that can be changed by atmospheric pollution – and it is more likely to contain beneficial elements such as nitrogen, an essential plant nutrient.  

Your rainwater might pick up some less beneficial additions, however. “If you are in a built-up area or an area of high pollution, I would recommend that you have a basic filter attached to your tank to remove potential contaminants like lead, which may otherwise end up concentrated in the soil,” cautions Adam. “Especially if you are growing food plants.” 

Continue conserving with a tap timer

You’ve done the hard work to collect all that precious rainwater, now it’s time to make sure not a single drop is wasted. Adam recommends adding tap timers to all your taps, tank or otherwise. “This is a simple way to conserve water,” he says. There are a few options to consider.  

Manual tap timer: This low-tech option has a mechanical dial dictating how long the tap will run, automatically switching off the water at the end of that period.  

Digital tap timer: A gadget that allows you to control both the duration of watering and when the water runs. This means you can schedule a dawn watering for your garden – to maximise absorption and minimise evaporation – without even getting out of your bed.  

Smart tap timer: This is a digital tap timer with more. Connected to an app, it gives you control from anywhere, with greater precision, and can be linked to the local weather service, so your irrigation system won’t switch on if rain is forecast. Even better, you can link it to compatible moisture sensors and smart weather stations, for a watering schedule that’s tailored to your garden’s unique needs.

Keep in mind

1. Contact your local council and water authority to find out whether you need approval to install a water tank on your property.  

2. Where a public water supply is available, health authorities generally recommend using it for drinking and cooking, and restricting rainwater strictly to non-drinking uses such as garden irrigation, flushing the toilet and washing clothes.  

3. Any plumbing or hardwired electrical work must be carried out by a licensed tradie.  

Want to live more sustainably?

For more eco tips, read our sustainability and recycling advice. 

Photo Credit: Alamy Stock Photo, Brigid Arnott

Health & Safety

Please make sure you use all equipment appropriately and safely when following the advice in these D.I.Y. videos. You need to be familiar with how to use equipment safely and follow the instructions that came with the equipment. If you are unsure, you may feel it is safest to consult an expert, such as the manufacturer or an expert Bunnings Team Member.

Grave health hazards are linked to asbestos, which may be in homes built up to 1990. Health hazards may result from exposure to lead-based paints in older materials and copper chromium arsenic (CCA) treated timber. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer. You can also use a simple test kit from Bunnings to indicate the presence of lead-based paint.