Background information

Why is the project needed?

In the older parts of central Auckland, wastewater and stormwater were designed 100+ years ago to flow through the same pipes called a ‘combined network'. While early last century this was considered innovative engineering as a response to issues with sanitation and odour when wastewater was disposed using ‘night carts’ were used. Over 100 years later and with population growth and stormwater flooding the wastewater network, the system is no longer suitable. Read more about the history of Auckland’s wastewater network.

During heavy rainfall, stormwater can overwhelm the combined network causing diluted wastewater overflows into our waterways and beaches such as St Marys Bay and the Harbour. These overflows are getting more frequent with population growth and extreme weather events. See: Why do we need to replace combined wastewater systems.

Modern networks and private drainage are now built to ensure wastewater and stormwater flow through separate pipes. Wastewater contains organic contaminants including bacteria and viruses that can make people sick so this needs to be contained and transferred to a treatment plant where it can be made safe before being discharged to the environment. Stormwater (rainwater) that flow off roofs and over land does not need to be treated to the same standard and can be discharged to the environment with little or no treatment. See: Stormwater and wastewater.
We've been working with Auckland Council on a major initiative called the Western Isthmus Water Quality Improvement Programme. This includes a range of infrastructure projects including the Central Interceptor and other local separation projects including this one in St Marys Bay. The project prioritises areas where beach and water quality is most affected.
In St Marys Bay, we plan to separate and upgrade the old combined network to today’s standard to reduce pollution from wastewater into the environment, particularly when it rains. To do this, we will build a new network of pipes and using the combined network so that there are separate pipes for wastewater and stormwater to flow through.

Refer to the map below which shows the general catchment area for St Marys Bay.