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

On-site wastewater treatment

On-site wastewater treatment is an option if there is no sewer available or if the owner wants to recycle water to reduce demand on mains wastewater systems.

The Building Code requires that if a sewer is available the drainage system must be connected to it – although the local authority can provide a waiver, allowing building owners to reduce demand on mains systems by recycling greywater or using a composting toilet.

If there is no sewer available, on-site treatment options will be needed.

With all wastewater disposal or recycling, health and safety must be the overriding priority.

Installing an on-site wastewater treatment system requires a building consent and the installation must be carried out or supervised by a certified drainlayer.

Some local authorities have specific requirements that homeowners must comply with. For example, Auckland Council requires that all wastewater treatment and disposal systems in its area are serviced at least every six months.

Blackwater and greywater

Blackwater is wastewater from toilets, dishwashing machines and sinks – because the fats, detergents and cleaning agents used in kitchen wastewater, this is considered blackwater and must be discharged accordingly.

Greywater is waste from baths, showers and hand basins.

Wastewater from clothes washing machines may be considered in either category.

Code requirements

An on-site wastewater treatment system must meet performance requirements for construction and operation of the Building Code. If the system is designed to AS/NZS 1547:2012 On-site domestic-wastewater management, the requirements of the Code will be met.

As noted above, the Building Code requires that if a sewer is available the drainage system must be connected to it – but the local authority can provide a waiver allowing on-site wastewater treatment to be installed. Local authorities or regional councils may have additional bylaws controlling the installation of on-site wastewater treatment systems. The local authority should be contacted before you begin to design a system.

The sanitary plumbing connected to an on-site wastewater treatment system within the house must comply with the requirements of NZBC clause G13 Foul water.

Septic tank construction is covered by AS/NZS 1546.1:2008 On-site domestic wastewater treatment units – Septic Tanks.

On-site wastewater treatment options

Most on-site wastewater treatment systems involve two stages of treatment – the first stage in a tank or treatment system, and the second when the effluent is dispersed on to land or the garden and further breaks down. The first stage may be carried out in a septic tank or in a more advanced system such as an aerated wastewater treatment system or advanced sewage treatment system. These systems are much more advanced than septic tanks and treat effluent to a level that allows it to be used on the garden or even recycled for toilet flushing and vehicle washing.

For new building projects, most local and regional councils require a higher level of wastewater treatment than that provided by the traditional septic tank.

Both of these options reduce the amount of effluent to be disposed of by the wastewater treatment system.

Investigate the power consumption and maintenance costs before specifying a model of sewage disposal system – these can vary greatly and can have a big impact on the ongoing costs for the owner.

Reducing water use

Installing water-saving devices in a house to reduce the volume of water going to the on-site system is recommended. Good options include:

  • WELS 3-star rated (or better) showerheads
  • WELS 5-star rated kitchen and bathroom tapware
  • dual-flush toilets (3/6 or 3/4.5 litre flushes)
  • front-loading washing machine rather than top-loading machine.

Reusing greywater in garden irrigation and, if treated, for toilet flushing, is another good way to reduce the volume of liquid going to an on-site wastewater treatment system.

Updated: 07 February 2023