Water supply tanks
The purpose of this document is to provide guidance information for building owners about the storage and
use of rainwater in water tanks.
1.
Using rainwater for potable purposes
The Ministry of Health recommends using mains water supply for the potable supply wherever possible.
However some properties, generally in rural locations, do not have an easily accessible public water supply
and therefore must use rainwater as a primary water supply.
If rainwater is used as the potable supply the water supply sytem must be
• Protected from contamination
• Installed in a manner that will avoid the likelihood of contamination within the water system
• Installed using components that will not contaminate the water
It is also recommended that the supply system undergo some form of treatment or filtration before
consumption to remove contaminants and that maintenance checks are performed on a regular basis such
as cleaning of the roof, gutters and tank and testing of the water.
A rainwater collection system comprises:
• Roof cladding
• Flashings and fixings
• Spouting and down pipes
• Tank material
2.
Do I need a building / resource consent to instal a water tank?
This depends on the capacity of the water tank and it’s height above ground; it also depends on whether it is
solely for the purpose of collecting water for reuse. If it is a dual purpose tank, it may require a resource
consent (refer next section)
Schedule 1 of the Building Act 2004 covers Building Work that does not require a building consent. Refer to
Exemption 23 copied below; anything outside this scope will require a building consent.
Page 1 of 3
October 2015
AC1821 (v.1)
Note: All building work must comply with the New Zealand Building Code, even if it is exempt from a building
consent. Furthermore, this work may require a resource consent; we recommended that you discuss your
proposal with Town Planners before starting work.
3.
What is a dual purpose tank?
A dual purpose or retention tank is where a portion of the volume of water stored in the tank is used for harvesting
(i.e. laundering, toilet or gardening) and a portion is used for detention (i.e. temporarily stores water to decrease
stormwater flows). In this instance the harvested water may only be used for non-potable use and must be plumbed in
by a certifying plumber to ensure cross-contamination does not occur.
If a dual purpose or retention tank is put in as a result of a condition of a resource consent, an encumbrance is
registered on the certificate of title to ensure the tank is retained and maintained on the site. If an owner chooses to
voluntarily instal a dual purpose tank an encumbrance will not be required. Dual purpose tanks require a building
consent if a public potable supply exists and the water is intended to be used for laundering, WC or gardening purposes
as there is a potential for backflow. An as-built plan showing the location of the tank must be provided on completion of
the work.
For further information about dual purpose tanks please refer to AC2264 On-site stormwater tanks, which can be found
on our website.
4.
Collecting rainwater
Some materials are not suitable for collecting drinking water; do not collect rainwater from the roof if there is
lead, chromium or cadmium in the materials. Lead could be present in the paint, flashings, nails, soldering,
cladding or any other part of the roof. Lead and chromium can also be found in primers and rust control
coatings.
Other roofing materials unsuitable for rainwater collection include but are not limited to:
• Any lead-based, asbestos cement or bituminous materials
• Wood treated with CCA (copper, chrome and arsenic) or LOSP (light organic solvent preservative),
e.g. timber shingles
Some roof materials and paints require washing or weathering before water collection starts to allow flushing
of any chemical residues that remain from manufacturing.
These materials include:
• Newly applied roof paint
• Galvanised steel
• Clay tiles
• Factory-painted steel or tiles
• Zinc
• Stainless steel
• Mineral fibre asphalt
• Slate
• Butyl (black butyl rubber will need regular maintenance and cleaning as it weathers)
Note: Only use roof paint that the manufacturer advises is safe for water collection; this information should
be indicated on the label.
5.
Other water supply sources
If water is drawn from a bore, spring or well and is intended for potable use, it needs to be tested to confirm it
is fit for purpose.
A sample of the water must be collected and tested by an IANZ accredited laboratory to DWSNZ 2005 to the
maximum acceptable values. A written report must be provided by the laboratory confirming the water is
suitable for consumption.
6.
Water tank
Tank materials need to be strong, durable and should not transmit light, as light will encourage organic
growth. An overflow outlet must be installed in the tank to prevent the tank from overfilling. The outlet should
be located and sized appropriately to cope with maximum inflow.
Page 2 of 3
October 2015
AC1821 (v.1)
Surplus water discharged from the tank overflow, or during cleaning, must go to an approved stormwater
system or soak hole.
Water tanks should be cleaned and inspected regularly. The Ministry of Health recommends annual tank
cleaning. Even with sediment control methods in place, sludge can build up in the bottom of the tank and
accumulated sediments can taint the water.
Note: ensure downpipes are disconnected if cleaning the roof or spouting
7.
Compatability of material
It is important to check the compatibility of the materials used in the rainwater collection system. If rainwater
is being harvested for potable use, in order to demonstrate compliance with Clause G12 Water Supplies
the roof cladding material must comply with
• AS/NZS4020 Testing of Products for Use in Contact with Drinking Water; alternatively a sample of
water collected from the roof can be tested by an IANZ accredited laboratory to DWSNZ 2005 to the
maximum acceptable values. A written report must be provided by the laboratory confirming the
water is suitable for consumption.
Certain materials when in contact with or subject to water run-off may cause corrosion or other undesirable
effects, even if they are individually suitable for water collection. If materials are incorrectly combined, severe
localised corrosion may occur and the collected rainwater may become contaminated.
A good source of information on material compatibility can be found in Clause E2 External Moisture of the
NZBC, Table 21 and 22. The tables, included in this document, show the compatibility of various materials in
contact with each other and materials subject to water run-off.
Page 3 of 3
October 2015
AC1821 (v.1)