How to choose a water tank

The cost of installing a rainwater tank can vary depending on the type of tank, installation and other requirements (like if you need a building permit). Whether you are building a new home or retrofitting a rainwater harvesting system, doing a little research can help you make the right choice for you. Here at, we’ve done the hard yards for you.


Consider where will you install your rainwater tank In an ideal world, your site would be...

If you are not sure if your site is suitable then give your local tank merchant a shout and see what they think. They might need to come and check your site.

Before you get too far ahead of yourself you’ll also need to consider the following:

What size water tank will you need?

Choosing the right size tank for your needs is often a decision based on what fits on your property versus how much water you need.

If your rainwater tank is your primary or single source of water then the general rule of thumb to get through the driest periods in your region is to get the biggest tank you can. This is especially the case if you live in an area that has unpredictable rainfall. Niwa predicts that the number of drought days we will experience will double in most regions in New Zealand over the next 20 or so years. So if you plan to stay where you are for a while, then the bigger the better might be the best solution to invest in future proofing your supply.

To work out how much water you need to ‘catch’ and store:

Work out the amount your catchment area can harvest by multiplying the roof area by the annual rainfall in your area...

Here’s the equation:

  1. Annual rainfall for your area A mm = A litres per square metre of catchment
  2. A litres per square metre of catchment multiplied by the catchment area B (roof area) = C litres of harvestable water


  1. Annual rainfall for Auckland (A) = 1212mm = 1212 litres per square metre of catchment
  2. 1212 litres (A) x 170m2 (B) roof area = 206,040 litres of harvestable water (C)

Now work out how much water you need e.g. garden use vs sole source of water


Family of 4 = 575 litres per day x 365 days = 295,875 litres per year If your catchment area x average rainfall doesn’t meet this requirement you will need to have a secondary source of water (town supply or purchase bulk water deliveries) or reduce your water usage, which can be done (see case study below).

Case study:

We have 2 adults and 2 kids living in our 188m2 house and one 25,000 tank in Mangawhai. With a total rainfall of 1025mm in the last 12 months we have easily survived without ordering further water (we don’t have town supply here). That would mean our water usage is much less than an average NZ family household however I don’t feel as though we try very hard. Yes we don’t leave the tap on when we brush our teeth like we did when we’re on town supply in Auckland but we still have an average 8 minute shower, an older top-loading washing machine (the most water sucking appliance in a house) and the kids have daily baths etc. We do need to be conscious of our water use in the drier months and had to top up with a 6,000 litre delivery in January 2017 when we had weeks without rain, but for $180 cost of delivery it’s still much less than what we were paying Watercare in Auckland.

What sizes of rainwater tanks are there?

Rainwater tank sizes

The largest manufactures of plastic rainwater tanks in New Zealand generally supply them in the following sizes: 1,000L, 2,000L, 3,000L/3,500L, 4,000L, 5,000L 9,000L, 10,000L, 13,500L, 15,000L 25,000L, 30,000L

The largest manufacturers of concrete rainwater tanks generally supply them in the following sizes: 1,100L, 1,800L, 5,000L, 6,300L 9,000L, 12,500L 22,500, 25,000L

Slimline water tanks: 1,000L, 2,000L, 3,000L, 4,000L, 5,000L

Choose the type of water tank that fits your needs

Each tank manufacturer will be working hard to convince you the material they use is the best. Whether its a concrete water tanks versus plastic tanks, each have their pro’s and cons. Plastic water tanks have become more popular over recent years however Kiwi’s and Aussies still love a good concrete tank. It often comes down to personal preference, price vs position - where will it go, does it need to be recessed or below ground or does it need to be easy to delivery and install.

Here’s a brief list of the good stuff and not so good stuff with the most popular materials in rainwater harvesting.

Concrete rainwater tanks

Good stuff

Not so good stuff

Polyethylene or Plastic rainwater tanks

Good stuff

Not so good stuff

Metal or Steel rainwater tanks

Good stuff

Not so good stuff*

*Stainless steel tanks mitigate these issues, however they are more costly

Fibreglass rainwater tanks

Good stuff

Not so good stuff

Treating your rainwater

Some governments and councils will only allow you to use rainwater for drinking, cooking and bathing purposes if it’s been filtered or treated. Check with your governing body for regulations.

Legal bits

In New Zealand you will need a building consent if you are connecting a rainwater system to the plumbing of a house that also has a mains supply. The rainwater system needs to be separate from the mains water by law which can be done by using a back flow prevention device installed by a qualified plumber. Even if you want to go completely off the grid, most councils will still require you to connect to the mains supply if it is available.

Click here for a full list of New Zealand councils website links and contact details. It’s a bit of a mine field navigating their sites to understand what legal requirements you need to adhere to so it might pay to pick up the phone and let them guide you in the right direction.

Article fine print

This is an independent article which means no manufacturer has paid to be featured or has influenced us writing about them in any way. Yay!