Septic tank failure in NZ: How serious is the problem?

This paper summarises a review carried out for the Ministry for the Environment on the current understanding of the incidence of on-site wastewater treatment system ‘failures’ in New Zealand, and the public health risks and environmental effects of such failures. While acknowledging the shortcomings in the available information, the following key conclusions were drawn:

1. There are in the order of 250 communities across the country with significant numbers of failing on-site wastewater treatment systems, mainly septic tank systems, comprising a total of about 42,000 houses.

2. Some of these failures would not be remedied by improved maintenance, since some failures are due to the siting of septic tanks in inappropriate locations (e.g. areas with high groundwater table).

3. Many failing systems will be affecting the localised area around the septic tanks, and nearby stormwater drains, presenting significant public health risks.

4. More than 100 streams and more than 100 coastal sites are potentially affected by failing septic tanks.

5. The number of groundwater systems that are potentially affected remains largely unknown.