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Getting the right rainwater tank for your needs

Rainwater tanks are available in a large variety of shapes, materials and sizes. Choosing the best water tank for your situation will help you to preserve one of our most precious resources and reduce your water bill. There are a number of points to consider when deciding which rainwater tank is going to be right for you.

Tank size and rainfall

The obvious decision you need to consider when purchasing a tank is around the tank size. A good rule of thumb is to purchase a tank that holds a minimum of four weeks supply. For instance, if you use 1,000L of tank water each week in the garden, toilet and laundry combined, you should consider purchasing a 4,000L tank.

Questions you may need to ask yourself could include:

  • How often will I be using the rainwater held in my tank?
  • How much rain does the area I live in typically receive?
  • How much space do I have around my home for a tank?
  • Will my tank require any structural support? (If your tank is going to be large, consult a builder or engineer as it may require structural support. You may also need a building permit from your local council)

How much rainfall will I collect?

Each square metre of roof area collects 1 litre of water for every 1 millimetre of rainfall received. This is determined by the area of roof connected to the tank via the stormwater down pipe. 

As an example:

  • An average house has a roof area of approximately 160 square metres.
  • Assume that 50% of this roof area can be easily connected to a rainwater tank i.e. 80 square metres.
  • Each square metre of roof area collects 1L of water for every 1mm of rainfall received.
  • If you have 10mm of rain, the water collected by the tank is 800L (80 square metres x 10mm = 800L).
Rainfall collection graph

Rule of thumb guide: roof collection versus tank capacity for an area receiving 600mm of rain per year.

Tank location

To reduce installation costs, you should locate your rainwater tank close to your house, near existing downpipes. Some of the options of positioning your tank are explained below.

Underground - Installing a rainwater tank underground can save space in your yard but can cost around twice as much as above ground options. They need to be made from stronger materials and structures and often require weighting or concrete reinforcement. Also, because underground tanks are positioned below ground level, you will need a pump to extract the water.

Above ground - The traditional round tank and some of the following space-efficient options are the most popular.

Under the house - For houses on stumps or stilts a space saving option is to put the tank under the house or beneath decking.

Against the wall - Another space saving option is tanks that go against walls or even act as garden walls. Some models can be joined together to make larger water storage tanks.

Tank material

Rainwater tanks are manufactured in a number of different materials. Before purchasing a tank, you should make sure you have the right tank for the right job.

Colorbond® steel - These tanks come with corrosive resistant coatings to prevent rust. There should be at least two metres of plastic pipe between a steel tank and copper, brass or bronze fittings as they can cause corrosion. Galvanised and Zincalume steel can be considered as an alternative, however they are both prone to rusting. Copper and stainless steel can also be considered for specific situations.

Fibreglass - Fibreglass tanks are durable and resist corrosion however, they are often more expensive and can expose the stored water to sunlight. Stagnant water exposed to sunlight can allow algae to grow, some forms of which may be toxic for both humans and pets. Ensure you purchase a tank which is manufactured with sufficient pigment to prevent light entering the tank.

Concrete - These tanks are strong and heavy which means they can be installed underground. New concrete tanks may need to be flushed clean because they can release excess lime leading to a high Ph in the water. Some concrete tanks require a lining to be installed.

Polyethylene - Plastic tanks respond well to bumps and are non-corrosive. These are often the cheapest tanks. Their advantage is that they are lightweight, easy to transport and are available in a range of colours.

Garden watering

Larger rainwater tanks tend to be more beneficial as you can store a large amount of water from the wet season to use in your garden during dry periods. Where space is an issue, you may wish to consider a different style of tank or installing two similar tanks.

Toilet flushing

A toilet can use up to 12L per flush. So installing a tank to a toilet system can lead to major water savings. An average person uses 30 litres of water in the toilet per day or 210 litres per week. You can install an automatic diversion directly from mains water if the rainwater tank is empty. Alternatively, a trickle top up system uses a float valve to measure how much water you have in your tank. When the water level gets too low, mains water will trickle into the tank to top it up. However, if the tank contains mains water it cannot be used on the garden during water restrictions, so this option should be considered carefully.