Network overflows

Dunedin’s wastewater network carries sewage to treatment plants at Tahuna and Green Island. During heavy rain, excess flows can get into some of the city’s waterways.

Sometimes during heavy rain, the wastewater network’s capacity is exceeded because stormwater or groundwater enters older pipes. To avoid wastewater backing up and being discharged onto private property or roads, outlets were historically constructed to direct overflows into the Kaikorai Stream, Lindsay Creek and Otago Harbour.

Overflows are not new, nor unique to Dunedin: they are an issue nationally and worldwide. We are committed to initially minimising and ultimately avoiding overflows through our capital works programme. Fixing the problem will cost millions of dollars and may take many years.

Where do overflows occur?

The DCC has been monitoring its overflows since 2012, and some operate more frequently than others. The University area, Sawyers Bay and South Dunedin overflows each operate around twice a year on average, the Kaikorai Common overflow operates two to three times per year on average, and the North East Valley overflow s typically operates six to nine times per year. The Kaikorai Valley Road overflow is the most active, operating on average 15 times per year.

What effect do the overflows have on the environment?

Our waterways are valued by the community for their cultural significance, recreational and aesthetic appeal,and the plants and animals they are home to. However, people’s activities have affected those waterways since the city was settled.

The overflows do not greatly change water quality indicators. This is because they happen during rain, when contaminants(including bacteria) are washed from the land into rivers and the sea. While ecosystems in the Kaikorai Stream, Lindsay Creek and Otago Harbour have adapted to this environment, we continue to work with the community and other organisations to improve water quality.

The effect of overflows on the city’s fresh and sea water has been investigated by an independent environmental consultant. Please see An assessment of receiving water environments in the related information section.

What effect do the overflows have on human health?

While overflows contain sewage, it is highly diluted by groundwater and freshwater that gets into the wastewater system, and by water flows in the waterways.

Regardless, bacteria and pathogens from the overflows are a risk to human health and we recommend people stay out of these waterways during heavy rain.

The overflows are relatively inaccessible (except at Kaikorai Common), so the risk of direct contact is low,and lower downstream as the overflows are further diluted by the waterway.

How often do overflows occur?

The DCC has been monitoring its overflows since 2012, and some operate more frequently than others. The University area, Sawyers Bay and South Dunedin overflows operate around twice a year, while the Kaikorai Common and North East Valley overflows typically operate six to nine times per year. The Kaikorai Valley Road overflow is the most active, operating on average 19 times per year.

What is the DCC doing about overflows?

We’re renewing the network to stop freshwater getting in and increasing pipe sizes where necessary. As we work through this process, overflows will be minimised. In the long term, the DCC wants to prevent the overflows occurring and we have identified this as a priority in our recent Integrated System Planning work - Integrated System Plan – Planning for the Next 50 Years

The cost of preventing overflows is estimated at $35–$130 million (2009–2011 figures). To balance cost and affordability, we’ll address overflows progressively  The annual spending on wastewater network renewals is increasing from $3 million in 2016 to approximately $19.4M million by 2025. The forecast total spend on wastewater network renewals over the next 9 years is approximately $180 million. We have also allocated $60M in our 9 year plan for wet weather flow management.

Work is currently focused in the Kaikorai Valley and North East Valley areas.

Our strategic and financial planning reflects our commitment to preventing overflows. See the 3 Waters Strategic Direction Statement, the Infrastructure Strategy and Dunedin’s Environment Strategy in the related information section.

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