Start a discussion

The Bunnings Workshop community can help with your home improvement projects.

How to choose a water tank

Valued Contributor



A water tank is a fantastic addition to your home to help reduce your environmental impact. Think of it as a rainwater harvesting, storage and distribution system.


Although there is an initial outlay, a water tank is an asset that lasts for decades while reducing water bills, so ultimately it could save you money as well as the environment. A tank can reduce the environmental burden of your home by reducing water wastage, runoff and the demands on civic infrastructure such as drainage. It also provides a degree of water security and in some cases means you may still be able to use outdoor water during times of restrictions. Interestingly, you might find that having a tank supply tends to make you more conscious of the rest of your water usage, too.


Here’s our guide to understanding rainwater harvesting and help with selecting the right tank for your place. We also have a seperate guide How to install a water tank


The simple structure


In a normal domestic home, rain falling on your roof is captured by the gutters, which transfer the water to the downpipes connected to an underground stormwater pipe system. This underground system might ultimately connect to the local stormwater mains or discharge to the street into the gutter through a kerb outlet.


74db3bd9-bc0c-459e-a9d5-4479adc851c8.pngWhen a rainwater tank is added, the water from downpipes is diverted to flow into the tank.
By adding a pump, the tank is connected to supply water to the garden or various household uses such as toilet flushing.

If the tank overflows, the surplus discharges to the stormwater system or a suitable dispersion system on your property.


The main parts of the system


In most circumstances you’ll connect (plumb-in) your rainwater tank to your existing system of roof gutter downpipes. For a simple connection your downpipes will likely only need a diversion added but there are often a couple of specialised pieces of plumbing in-between gutter and tank. There are different types of pipe installations too and we’ll look at those in detail in the section on connecting.

A rain-head filter sits below the very first stage of your gutter outlet. These have a wide-mouthed hopper type shape and an angled mesh screen over their open face. Any leaves or debris that flush through from the gutters will slide off the sloped mesh as water goes through the mesh into the hopper and down the pipe to your tank.

Many systems will have a device, often called a first flush diverter, between the gutters and the tank inlet. These are designed to redirect the first run of a downpour so that any potential dust and contaminants from your roof are collected and discharged into the garden or stormwater system.


The inlet opening on top of the tank will have a mesh screen. This is not just for keeping leaves and debris out, it is to mosquito-proof your tank and to keep any animals from falling into the tank.

3d19bdcf-c4ef-4ea7-a68e-d645c2f57be9.jpegThe tank itself may be corrugated steel or heavy-duty, UV stabilised, food-grade polypropylene (generally just called poly). With either there is a huge range of shapes, sizes and colours available. Concrete tanks are also an option but these are most often installed in-ground and most domestic retro-fits are above-ground.


Your tank needs to have a demand pump connected to its outlet. This does not need to be right beside the tank but needs a suitable power outlet. These pumps sense a tap opening and (virtually instantly) turn on, supplying water. If you plan to have your tank supplying water to fixtures such as toilets and laundry then you’ll also need an automatic mains water switch valve fitted. These recognise when the tank runs dry or when power is cut and automatically change over to using mains water supply to ensure your fixtures still have water supplied.

Your tank will have an overflow outlet. This should be connected to the stormwater or be discharged on your property in a way that complies with appropriate council and state regulations and likely needs some form of mosquito screening on it.


Evaluate your needs and possibilities


How would you like to use your tank water? Just for watering the garden? Or do you want extra taps for washing the car and topping-up the swimming pool? You may want to go to the next level and have it connected to internal fixtures, supplying toilets for flushing and the cold-water inlet on your washing machine.


In many cases the best first step is to check with your local council to see if they have any restrictions on size or location of a tank and the uses of rainwater.


In some circumstances a development application may be required. If you plan to connect to toilets and laundry, then plumbing approval will be needed. Bear in mind too that when connecting to internal fixtures you need to also connect the mains supply to the pump via an automatic mains water switch valve to ensure continuity of water supply should your tank run dry or power be cut.


How much rainwater can you potentially harvest?


In a domestic retro-fit, the tank will usually be connected to only one or two downpipes based on what can be conveniently achieved, with the least visual impact, in the location of the tank.


To find out how much water you can potentially capture, and therefore help you estimate best tank size, you need to know two things – the approximate size in square metres (length x width) of the roof area above the gutter and downpipes you’ll be connecting to, and the average annual rainfall for your area. You should find that the BoM webpage or your local council can provide this information.


You then need to perform some maths. 1mm of rain falling on 1sq/m of roof = 1L of water. So, if your measured roof area is 40sq/m and your average rainfall is 700mm then around 28,000l of water a year runs across that roof. (40 x 700 = 28,000).

A couple of points to consider are that rain capture is never 100%. By the time you account for things like splashing and gutter overrun it’s probably closer to 80% so, to continue with the same figures above, your 28kilolitres ends up closer to 22kl.

Another important point is that in most areas rain is not evenly distributed throughout the year. Increasingly we are seeing large downpours followed by drier periods. This means it’s likely that you’ll experience overflow and then nearly running dry before your tank is topped-up again.


How large should a tank be?


Much of this decision comes down to how much space you have and how much roof you can connect. The experts say that the genuinely useful minimum is 5000L, while 8000L is the minimum if you connect to toilets. But don’t let these minimums put you off. Even just a 1,000 or 2,000l tank can be a real garden lifesaver over hot and dry periods.

15306027-ff1b-4e82-936a-7cdaf009d2e6.jpegWhen it comes to capacity, the bigger the better. It’s not that much more expensive to jump up in size by a couple of thousand litres, so generally go for as big as space allows, although there’s obviously little point in connecting a 22,000l tank to a roof that only sees 15,000l of water a year.

The way you decide to connect your tank can also make a difference to the size as this changes the amount of roof area your tank can service. Take into account the points covered in the next two areas too.


Selecting a tank


The first step is to contact your local council and find out if there are any restrictions on where you can locate a tank on your block, the size you can install, tank colour or construction materials. If you are using a specialised installer they should either know or check this information for you.

Now look at available tank dimensions, materials and colours. You’ll find the range of colours is such that a tank can very easily be camouflaged in virtually any space.

Ultimately the material you choose will likely come down to availability in the sizes and shapes you need and your budget.

Poly tanks are available in a set range of sizes and shapes. Corrugated tanks can also be customised to suit your needs and space.


An important, often-overlooked consideration is site access. Selecting a 1.2m wide tank when your garden gate is only 800mm wide can pose major problems. Yes, in many cases you can have a tank lifted in by crane, however this will add a substantial amount to your install costs.

In some circumstances you may find you need to connect a couple of smaller tanks to get the capacity you need. This is easy enough to do however you do need to select tanks that are suitable for “daisy chaining”.


Positioning and connecting your tank


Some of the decisions about the position of your tank may be made for you by any number of the points covered in tank selection.


This then brings you to the all-important area of connection. The more roof you can connect the better so this may govern your final decision on tank location and of course, size.

There are two main types of pipe connection:


connection.png1. Gravity lines. This is where your tank is directly connected to the existing downpipes and water travels to the tank by running down the pipes. You would use a gravity line where you only connect to one or two downpipes.


2. Charged lines. These are used when you want to connect more of your downpipes or your tank is located away from the house. The pipes are installed underground and remain filled with water at all times. Rainwater then enters the pipes from the gutters and pushes existing water out the other end into your tank. The pipe discharging into the tank must be around 500m lower than the lowest roof gutter it is connected to avoid a backing-up effect.


A charged line can be used for new lines or if you tap into the existing stormwater pipes below ground. If your main output can be located you may even be able to connect your entire roof area to your tank, massively increasing your rain harvesting potential. If you do wish to take this more complex approach you’ll need to enlist the help of a professional.


Supporting your tank

Water tank.pngTanks need very stable support. It is very easy to underestimate how much weight is in a filled tank but here the very neat metric system makes it easy for us again.


1 litre of water weighs 1 kilogram, therefore every thousand litres will weigh 1,000 kilograms or 1 tonne. Your 10,000-litre tank? It will weigh in at over 10 tonnes once full!


What this means is you can’t just plonk a tank down on the lawn and expect it to not settle.

Often a reinforced concrete slab is laid, however a cheaper option is to just use a compacted gravel and roadbase bed. Whatever you choose it will need to be completely level.


What professionals do I need?


There are various parts of the installation of a water tank you can do yourself depending on your skill level. You will however need to bring in a plumber for stormwater changes and connections, new water pipes and taps and (if needed) internal fixture and mains water connection.


If you don’t already have an outdoor power point you’ll need an electrician, too.


You may find it simpler to engage a water tank specialist as they can organise all levels of work and any council approvals you may need.


Taking all of this information on-board will help you make the right decisions when selecting a rain harvesting system. It will ensure you have a system that will suit your needs for years (if not decades) to come.


Need help installing your new water tank? Check out our step-by-step guide How to install a water tank


Sponsored Post

Why join the Bunnings Workshop community?

Workshop is a friendly place to learn, get ideas and find inspiration for your home improvement projects