For most people a rainwater tank is an investment in the future. As we don’t yet pay the true cost of water delivered to our door, it will take many years for a rainwater harvesting system to break even.

To make the most of your investment, plumb your rainwater tank so you can use the water inside your home. There are big advantages in terms of efficiency, even though the water in most tanks won't last through the summer. Household appliances that use water frequently, such as your washing machine and toilets, will continually draw down the water level in the tank and create more storage capacity for the next rain shower. That's good! Because the winter rains keep refilling the tank, you will re-use the water tank capacity several times over a year.

A 3000L slimline tank could easily yield 30,000L of water if supplying toilets and laundry. A 5000L tank could supply 75kL or more of water to toilets, laundry and hot water system – 15 times the capacity of the tank!

In terms of saving water:



Typical Installation

Cost per litre

Good Collecting rainwater for the garden 1500L tank, no pump 30c
Better Plumbing-in your laundry and toilets 3000L slimline tank, submersible pump 15c
Best Also supplying your hot water system 6000L slimline with pump 6c
Little better Adding further tank capacity e.g. another 3000L tank 5c


”Going Large” ...putting in a larger rain tank

Is it a good idea to have a bigger tank? It depends a bit! If you're using the tank primarily for watering your garden, then the bigger the better. You'll be lucky if it refills through the dry summer, so the more water you've saved up by the time it gets hot, the better off you'll be.

If you have the space, remember that round tanks cost around 2/3 that of slimlines (due to the lower manufacturing cost)

In the metro area most rainwater tanks connected to the home will run out in October / November time - within a few weeks of the end of the winter rain – because of their limited capacity. Making your tank a little bigger (e.g., going from 6000L to 9000L) has minimal impact on the overall efficiency of the system. Unless you live on a rural block and have space for a large tank that will hold enough rainwater to last through most or all of the summer (e.g. 90kL or more) you should plan on your tank water only lasting 5-7 months per year.

What happens when the tank runs dry?

Through the dry part of the year your house will continue to be supplied with mains water. This is normally done through an automatic changeover device mounted on an outside wall which will supply tank water if available, without requiring any valves or taps to be turned on and off manually.

Using greywater indoors

It is possible to use greywater (the waste water from your showers or laundry) to flush toilets inside the home. You’ll need a Greywater Treatment System (GTS) to do this, which purifies the water sufficiently for it to be used safely inside the home. These systems are going to cost you 3-4 times as much as a plumbed rainwater tank, and need regular servicing thereafter. Unless water is in very short supply we don’t recommend use of a GTS.

For the most cost-effective water savings, plan on using rainwater inside your home, and greywater on your garden through a simple Diversion Device (GDD). See the Greywater Page for more info.
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