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Dunedin City Council – Kaunihera-a-rohe o Otepoti

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?

Dunedin has six overflows in:

  • Kaikorai Valley (Kaikorai Common and Kaikorai Valley Road, affecting the Kaikorai Stream)
  • South Dunedin (affecting the upper Harbour Basin)
  • North East Valley (affecting Lindsay Creek)
  • University area (affecting the upper Harbour Basin at the mouth of the Water of Leith)
  • Sawyers Bay (affecting Otago Harbour).

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.

The cost of preventing overflows is estimated at $35–$130 million (2009–2011 figures). To balance cost and affordability, we’ll address each overflow in order. The annual spending on wastewater network renewals is increasing from $3 million in 2016 to $6 million by 2025. The forecast total spend over the next 10 years is $48 million.

Work is currently focused in the Kaikorai area.

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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