Rainwater tanks

An installed water tankRainwater tanks

Many urban Australians have been installing tanks to catch the rainwater that falls on their roofs and in recent years, and Government subsidy schemes have been in place to encourage their installation.

Past studies of the hydrology of rainwater tanks in urban areas often assumed that they would be used to supplement garden watering. They showed that the size of tank needed to store water from wet periods for use in the dry made the exercise uneconomic. However, there has been a change in thinking in more recent years with tank water now used for toilet flushing, clothes washing and even hot water supply for general use. These are relatively constant uses throughout the year and, with the reticulated supply as an automatic backup, it is possible to significantly reduce the demand on the metropolitan system even with modestly sized tanks (eg. 5 kL).  It is perhaps advisable for households to examine rainwater harvesting in association with possible greywater re-use.
If it is possible to re-use from both sources, this can be ideal for households as greywater can help stretch the use of the higher quality rainwater and save it for particular uses for which greywater may not be ideal or may have additional complications.   

Interception of roofwater through harvesting via rainwater tanks can have significant benefits such as:

1. Social and Environmental Benefits

Harvesting your rainwater will help in reducing volumes of stormwater discharged to local waterways and, if done by enough people in the catchment, can even reduce the peaks of flood events.

Rainwater harvesting will also help reduce pollutant loads transmitted to streams as what would otherwise become stormwater is captured as soon as it comes off the roof and prevented from entering streets where it would collect pollutants and transport them to waterways. Instead, where a tank is employed, atmospheric nitrogen for example – an important component of nutrients in rainwater – as well as any other pollutants and sediment that come off the roof, are captured on-site. The nitrogen component of rainwater can actually assist with plant growth when applied in the garden.

If tanks are widely employed, although pollutants in stormwater will be more concentrated due the reduced amount of diluting stormwater, the volume to be treated is reduced and therefore should be less expensive. 

Rainwater tanks need to have a connection to stormwater for overflow in times of high rainfall.  It’s possible to employ a raingarden to treat overflow from a rainwater tank prior to discharge to the stormwater system.  This is achieved via the sandy media used in raingardens.  The media can filter sediment, while plants can take up nutrients, thereby ensuring that any overflow is in the best possible condition when it is discharged to stormwater.  See:

2. Householder Benefits

From an individual household point of view, collecting rainwater from your roof can result in significant water and cost savings. These will increase over time as potable water moves to more than $3 per kilolitre (from the current $1.71 as at 2011) in the future. Importantly, it will also contribute to reducing the overall demand for mains water, as a number of in-house uses are replaced with rainwater. All garden irrigation can also be done with rainwater, or greywater, with the latter being preferable if this is possible. Given that outdoor water use accounts for around 20% of total use and toilet flushing around 14%, replacement of just these two uses can make a sizeable difference to household water use and provide some significant cost savings especially if initial capital outlay are not excessive .  

In combination, rainwater harvesting and greywater re-use has been demonstrated to be highly effective in replacing use of potable or mains water.  A leading example of domestic water conservation in Melbourne comes from the author of Water – Not Down the Drain (McQuire, 2008), where, through investment in a number of water saving devices and storage systems, a four person household in a Californian bungalow in West Brunswick was able to achieve a 96% reduction in water use compared to the average Melbourne household and used just over 5% of potable water during 2007/8 – and that at a time of severe drought. 

Rainwater Regulation and Health Considerations

‘Rainwater’ is used to describe the water that falls on your roof when it rains. It does not include rainfall that is collected from the ground, which is usually called ‘stormwater’. Unfortunately, rainwater can be contaminated from a number of sources. These include roof materials, animals (possum poo) and birds, leaves, debris and emissions from wood heaters. However, roofwater generally has much lower levels of chemical contaminants and pathogens than urban stormwater.

The relevant national guidelines for rainwater state the following:

“The decision about how to use rainwater is a matter of personal choice. In making this decision, it should be recognised that, although the risk of contracting illness from rainwater supplied from well-maintained roof catchments and tanks is low, the quality of water from household tanks is not as consistently high as that provided by well-managed urban water supplies. Microbiological quality is not as reliable as mains water, particularly after rain events. In addition, there are a few areas where impacts from major industrial emissions (for example, Port Pirie, South Australia) mean tank rainwater is not suitable for drinking and food preparation. The impacts on rainwater of very large densities of traffic, and other emissions, in Sydney and Melbourne are yet to be determined.  

One option to decrease any potential risk from tank rainwater is to minimise oral exposure by limiting use of the collected water to supplying hot water services, bathing, laundry, toilet flushing or gardening (that is, not for drinking or food preparation).  

The water quality requirements for non-potable uses are lower than those for drinking water. Guideline values cited in the Australian drinking water guidelines are based on a daily consumption of 2 L of water per day for an adult and 1 L for a child. Consumption or ingestion from showering is generally less than 100 mL per day. Ingestion associated with laundry use, toilet flushing and gardening would be even lower.

Use of rainwater to supply hot water services has attracted increasing interest. From a practical economic view, hot water supplies involve separate plumbing systems, so it is relatively easy to incorporate the use of rainwater in a house for this purpose, with minimal duplication of pipes or fittings. In hot water systems, risk from rainwater is lowered by a combination of reduced exposure and thermal inactivation of enteric pathogens (microorganisms that cause gastrointestinal illness). Drinking water from hot water services is generally not recommended.”

[Guidance on Use of Rainwater Tanks, EnHealth, Australian Government, 2004.]

The Victorian Department of Human Services publication Rainwater Use in Urban Communities- Guidelines for Non-drinking Applications in Multi-Residential, Commercial and Community Facilities (2007) states the following. 
(It should be noted that these Guidelines apply to community-type facilities and are not intended to specifically cover private use of rainwater)

The quality and acceptable uses of rainwater are not subject to specific regulation in Victoria. Despite this, individuals or organisations responsible for rainwater systems in urban communities should demonstrate due diligence by ensuring that rainwater is safe for its intended use. The quality of rainwater and the associated management controls need to be proportional to the level of exposure to rainwater - the more likely it is that rainwater will be ingested, the higher the water quality and more stringent the management controls need to be. 

The Department of Human Services recommends that rainwater is not used for drinking and food preparation in areas where a water authority supplies reticulated (or "mains") drinking water. This is because the quality of rainwater is generally not as reliable as mains supplies, which have been treated to a level to ensure they are safe for human consumption.” 

The Guidelines further state that: “as rainwater collected from well-maintained rooftops is generally of a quality that is safe for many uses, the need for treatment is unlikely. However, in some circumstances there may be a need to treat rainwater.” 

 And further: “Whether the water is treated or not, a full rainwater system inspection should be undertaken at least quarterly.”

 The Australian Rainwater Industry Development Group has published information that sets out where rainwater can be used in the home and includes garden (surface and sub-surface) irrigation, toilet, laundry, hot water, and cold water to the shower. See this EPA publication for further information. 

Developing a Rainwater Storage System

Rainwater storage tanks are widely available, either through water retailers or tank companies and come in a wide range of sizes, shapes, colours and materials.

Installation must be carried out by a licensed plumber (Green Plumbers website), and tank systems must be independent of the mains water supply to avoid contamination. Water tanks must also have an overflow pipe connected to your existing stormwater system to manage overflows from large rainfall events.

The type of tank you choose will depend on:

  • Available space on your property and the fit of the desired tank(s). As many homes, and especially a large number within the City of Yarra, have limited areas available for rainwater tanks, this is a critical factor. As a guide rainwater tanks with a capacity of 5000 litres will occupy an area of about 2 m2 and a tank with a capacity of 15,000 litres will occupy an area of 6 m2.
  • It should be noted that tanks now come in many materials and shapes including under-building squat tanks that can fit below deck areas. Tanks can also be placed adjacent to side fences or they can be incorporated within structural walls. Tanks can also form part of the landscaping of backyards. It is also possible to have tanks incorporated into the concrete slabs of new homes
  • Roof size. One simple method of calculation is to multiply your roof area (less a factor for any area not possible to be connected to storage) by your suburb’s rainfall (the average for Melbourne is about 650mm, with some areas just north of the CBD experiencing slightly more). A deduction should also be made for loss associated with first flush in every rain event.  This will provide your maximum storage capacity as each m2 of roof area collects 1 litre of water for every 1mm of rainfall received. See the Savewater website. An average double-fronted house in City of Yarra will probably have a roof area of around 160-170 m2.and a single-fronted house possibly around 70-80 m2.
  • The following link takes you to a 2008 Victorian Government publication that on p. 7 indicates a typical breakdown of water use in a Victorian home.  How many people are there in the house and how many uses will rainwater be put to?  Use your water bills from City West Water or Yarra Valley Water over the last year to help with your assessment. Toilet flushing, washing machine and outdoor use are the most common uses for stored rainwater, but there may be other uses for which storage is required (eg. dishwasher, cold water to shower and/or bath). 

The following provides some guide to domestic water use for items that are readily quantifiable. 

  • 3 litres for half flush and 6 litres (or 4.5 is now available) for full flush on modern dual flush toilets (11 litres for the original single flush cisterns)

  • 90-110 litres for top loading washing machines and 50-80 litres per wash for front load machines

  • around 15 litres per minute for garden hoses. 

The greater the reliability required, the more uses to which rainwater is to be put and the greater number of residents in a household, the larger the size of the tank required for rainwater to serve as a regular replacement for potable water.  In addition you may want to factor in an additional 20% if you don’t wish to regularly use the last part of your stored water due to its smell or concern over sediment contamination.

  • The extent to which rainwater and greywater re-use will be employed in combination to meet household needs and replace potable water.  This is likely to depend on whether a greywater treatment system is employed.  While these systems allow storage and allow re-introduction of greywater into the house for toilet flushing and clothes washing, the economics of using such systems is highly unattractive.  However, if a householder was to pursue that option, it would mean that more rainwater could be directed to higher quality ends within the home such as for the dishwasher, cold water to shower and bath and even perhaps hot water. 

 Tank Sizing

There are a number of methods available to assist with tank sizing.  They include:

  • the ecowho calculator - this requires input of rainfall data but is otherwise quick to calculate
  • the University of South Australia calculator. This is an Excel based spreadsheet
  • Set up your own Excel spreadsheet and follow the methodology outlined by Cunliffe (1998) 
  • the simplest one is The Tankulator developed recently by the Alternative Technology Association and funded by Sustainability Victoria.  (Tip - play with various settings and questionnaire inputs to ensure you have covered your circumstances.) 

As a general guide, even on small single-fronted properties, on the basis of economics alone, it would not make a lot of sense to install a rainwater tank that has less than a 4000 litre storage capacity as economies of scale do play a role. This might be particularly so if plumbing to toilet flushing or other uses needs to be retrofitted thereby increasing the cost – although rebates (see later section) may help offset some of the cost of the latter. On the other hand, where residents have a double-fronted property and space is available, many households with a keen interest in replacement of potable water and who connect rainwater to a number of uses would possibly prefer to consider a tank size between 10,000 and 20,000 litres as this may deliver greater reliability (see further below), allow less overflow in exceptional rainfall events and have a shorter pay-back period.

Council Requirements

To find out if you need to obtain a building and/or planning permit to install your rainwater tank, please click here.


If a licensed plumber installs a rainwater tank sized between 2000 and 3999 litres and it is connected to laundry or toilet you will be eligible for a $500 rebate.  If you install a tank with a size upwards of 4000 litres connected to laundry or toilet you will be eligible for a $900 rebate.  If you install a tank with a size upwards of 4,000 litres connected to both laundry and toilet you will be eligible for a $1000 rebate.  If you have an existing tank of 2000 or more which has not previously been connected to toilet or laundry and you install those connections you will be eligible for a $200 rebate. Click here for more information about rebates.

Recommended Reading

McQuire, S. Water – Not Down the Drain, CSIRO Publishing, Collingwood 2008

For any Yarra Residents with a desire to investigate domestic water conservation further, Stuart McQuire’s book is highly recommended and very relevant to a Melbourne context, although written with a national audience in mind.  Despite being a few years since initial publication, it is still full of useful advice and information, along with insights into his own inspiring story of domestic water saving.  Copies may now be more difficult to obtain, but can be sought from CSIRO Publishing at 150 Oxford Street, Collingwood (ph. 9662-7555) or from the Alternative Technology Association (ph. 9639 1500 or Email: ata@ata.org.au).

Further information
David Taylor
Water Management Officer
9205 5777

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