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Alternative water: rainwater

Using rainwater captured from roofs is a popular water supply for many South Australian homeowners. Roof rainwater may be used as the sole source of water (including for drinking) when mains water is not available, or a secondary source of water supplementing mains supplied water to reduce mains water use and bills.

As a supplementary supply, roof rainwater can reduce the amount of water that would otherwise be taken from other sources, such as the River Murray.

When considering large-scale use of roof rainwater, it is necessary to consider:

Whether roof rainwater is the only source of water or a supplementary source of water.
The reliability of roof rainwater to provide water in dry seasons and during droughts. Climate change may increase the risk of relying on rainwater and necessitate more careful management of roof rainwater supplies.

Potential advantages and disadvantages of roof rainwater versus other water sources (e.g. water quality).

Rainwater use

Unlike metered water supplies such as the mains water supplied by SA Water, domestic rainwater tank use is not generally metered. This makes it difficult to be sure how much rainwater is collected and used from year to year, or seasonally, across South Australia. Information about rainwater tank use can therefore only be approximate.

In Adelaide, it is estimated that an annual average of approximately 1.5 GL per year of roof rainwater may be used. This suggests Adelaide’s domestic rainwater use is nearly 1% of Adelaide’s average annual water use.

Rainwater use is higher outside of metropolitan Adelaide, particularly in areas not connected to the SA Water system. In the Mount Lofty Ranges, it is estimated that domestic rainwater use may be around 1 GL.

In the Surface Water Prescribed Areas within South Australia, roof runoff greater than 1500 kilolitres per annum collected from a connected roof area and used for commercial (including irrigation), industrial, environmental or recreational uses must be licensed.

Water quality

While rainwater has a lower salinity than most other water sources, it is not necessarily free from contaminants, and may contain micro-organisms, dust or other pollutants flushed from building roofs and gutters. For this reason many people prefer to use rainwater for non-drinking uses such as garden watering, washing clothes and flushing toilets. Unlike mains water supplies, rainwater does not contain fluoride which plays an important role in protecting against tooth decay.

SA Health recognises that if rainwater collected from a roof is clear, has little taste or smell and is from a well-maintained system, it is likely safe to drink and is unlikely to cause any illness for most users. South Australian policy supports people making their own decisions about whether they choose to consume roof rainwater - although it is recommended that drinking roof rainwater be avoided in areas affected by very heavy traffic, industry, incinerators and smelters.

Rainwater in Port Pirie contains lead and SA Health recommends that you do not drink the rainwater, use it for food preparation, or water edible fruit and vegetables. (See the SA Health Rainwater in Port Pirie web page).

Rainwater tank users should make themselves aware of possible water quality issues and the importance of tank maintenance. Further information is available from the SA Health Rainwater web page.

Climate change

Climate change will affect the seasonal and inter-annual variability of rainfall as well as the geographical distribution. These changes will affect the availability of roof rainwater resources. In South Australia, the average annual rainfall will decline. Spring rainfall declines will be greater than any other season. Time spent in drought will increase and the number and intensity of heavy rainfall events will increase. (See the South Australian climate projections).

Could Adelaide rely entirely on roof rainwater?

While roof rainwater is a water source for many South Australians, it could never completely meet the water needs of most water users in Adelaide or other towns or cities, without requiring people to:

significantly reduce their water use which may severely impact their existing lifestyle
manage their rainwater on a day-to-day basis to ensure enough rainwater is available in low rainfall seasons and prolonged droughts

regularly maintaining their rainwater systems to ensure that water quality is ‘fit for purpose’.

To collect as much rainwater as possible, every home would need to connect their whole roof to as large a rainwater tank as possible. These large tanks would necessitate lifestyle compromises, particularly for those living on small blocks who may already have little, or no, room available to install a large rainwater collection tank.

A 2004 desktop review of rainwater tank options for Adelaide households estimated that even if the installation of plumbed in-house tank supply was mandated when homes were sold, rainwater tanks would only supply around 10% of Adelaide’s water. At the time, the initial capital cost was estimated to be about $2000 per dwelling ($3433 in 2024), with an estimated cost in the order of $5.60 per kilolitre ($9.61 per kilolitre in 2024).

Despite the relatively high cost of rainwater, this source of supply can offer advantages, including:

Rainwater tanks can provide a short-term back-up supply if problems arise with the normal water supply (e.g. if a mains water connection is cut off while maintenance or repairs are undertaken).
Rainwater is lower in salinity compared to other water sources, making it a great source of water for salt-intolerant plants.
Using rainwater as a supplementary supply can reduce the amount of water taken from other sources such as the River Murray.
Research indicates that many buildings capturing and using roof runoff may help to reduce the risk of minor street-level flooding, which will be more important as urban density increases and heavy rainfall becomes more intense due to climate change.

Research in Perth found rainwater tanks to be associated with increased residential property values by an amount greater than the up-front cost of the rainwater tank. This suggests some home buyers might be willing to pay a premium for rainwater harvesting systems as a desirable housing sustainability feature.

State planning policy support

South Australia’s Planning and Design Code facilitates facilitate rainwater tanks being considered when planning some types of new developments. For example, a mandatory rainwater tank policy exists for new homes and some extensions to existing homes built after July 2006 and for dwellings that will not be connected to a reticulated or mains water supply, provisions allow for dwellings to be serviced by a rainwater tank or tanks of at least 50,000 litres capacity.

Provisions for stormwater management also support rainwater tanks through:

On-site stormwater retention - rainwater tanks that capture runoff and are connected to certain water-using areas of the dwelling
On-site stormwater detention - rainwater tanks that provide temporary storage of roof runoff with gradual release from the property – the aim being to reduce the rate of runoff from the dwelling so that it doesn’t adversely compromise the public stormwater drainage network that runoff from the development discharges to.

Further information can be found in an Options Analysis paper by BDO Econsearch that informed the rainwater tank provisions introduced in the Planning and Design Code.

More information

Rainwater tank information and advice

Guidance on the use of rainwater tanks (water quality focus)

Studies that have looked at the cost effectiveness of rainwater tanks

Climate change projections

Guide to climate projections for risk assessment and planning in South Australia 2022

Volume	Conversion	Description
Gigalitre (GL)	1GL = 1 billion litres	One billion litres of water weighs one million tonnes.
Megalitre (ML)	1 ML = 1000kL or 1 million litres	Adelaide currently uses around 200 GL of water per year from SA Water supplies, groundwater and other sources.