Water use, sustainability, and efficiency by choosing quality systems and materials, and providing environmentally friendly solutions.

Mains or rainwater?

Most buildings will use mains water supply. However, rainwater is an option to partly or fully replace mains water.

On this page:

  • Building Code requirements
  • Ministry of Health recommendations
  • Designing a rainwater system

Mains systems offer reliable supply that meets required standards for potable (drinkable) water.

For most buildings where mains water is available, using the mains supply will be the most effective way to meet users’  water needs. Other features – such as specifying efficient fixtures and appliances, and designing an efficient water heating system – can be used to reduce the building’s environmental impact.

But rainwater can be used to provide water supply when there is no mains connection available. Even if there is a mains connection available, rainwater can be used either to meet all of a building’s demand for water (depending on users’ water needs) or to meet some water supply needs such as providing water for gardening, flushing the toilet, and/or bathing and clothes washing.

Using rainwater can:

  • reduce costs for users on a metered water supply
  • provide an emergency supply
  • reduce demand on mains water supplies
  • reduce demand on stormwater disposal systems
  • provide an independent supply for watering the garden in times of drought
  • reduce the rate of stormwater runoff.

For all its benefits, harvesting rainwater brings maintenance requirements that do not apply to mains water supply. Specific maintenance tasks should be pointed out to clients considering this option, without discouraging them, before they make a final decision.

In New Zealand, approximately 10% of the population depends on roof-collected rainwater for drinking water.

Building Code requirements

Any rainwater system must meet relevant Building Code requirements.

This includes a requirement for adequate potable (drinkable) water to be provided for consumption, oral hygiene, utensil washing and food preparation. This potable water supply must be protected from contamination, and must not contaminate the water supply system or source.

The Building Code also requires adequate water supply to any sanitary fixture (such as toilets, baths, showers, sinks and so on). The sanitary plumbing system must be set up to minimise any risk of illness or injury.

If collected rainwater is piped into a house and/or connected to a mains supply system, a building consent is required and the installation must be carried out by a certifying plumber.

Rainwater storage tanks (whether plumbed to the house or not) also require a building consent where they: exceed 2,000 litres capacity and are supported not more than 2 metres above the ground; or exceed 500 litres capacity and are supported not more than 4 metres above the ground.

Any rainwater system that is connected to a mains water supply must be designed to minimise the risk of contamination of the mains water supply by including an air gap or backflow prevention device. Acceptable Solution G12/AS1 requires that the air gap must be the greater of 25 mm, or twice the diameter of the supply pipe. The system must also be designed to minimise the risk of contamination to rainwater intended for household use.

Ministry of Health recommendations

The Ministry of Health recommends that:

  • where mains water supply is available, it is used for drinking and food preparation
  • where no mains supply is available, a water filtration system and/or treatment for drinking water should be specified.

Designing and installing a rainwater system

The design and installation of any rainwater system will depend on its purpose – whether it is a small rainwater barrel used for watering the garden, or a larger storage system for sanitary uses or potable water supply.

In general, rainwater will be harvested from the roof and stored in a tank until use. The system must be designed, installed and used to minimise the risk of contamination.

Irrespective of whether mains or rainwater is used, the designer will need to consider how to manage water pressure, and pipework layout and specifications (see system design and pipework), as well as how to achieve efficient water heating.


Updated: 24 January 2023