The risk of emergency overflows of sewage into the Manukau Harbour for a further 50 years is being highlighted by community groups opposed to the proposed Auckland Central Interceptor waste tunnel.
The 13km tunnel will store polluted storm water and sewage from as far afield as Western Springs to reduce overflows into the Waitemata Harbour nearly every time it rains.
The tunnel will leave the isthmus to go 40m under the Manukau Harbour to the shore at Mangere where a new pump station will lift the contents out of the main tunnel and send them to the nearby Mangere Treatment Plant.
Watercare Services proposes that the pump station have an emergency pressure relief structure which can be activated to stop a full tunnel overflowing at other shafts on the route.
Objectors included the Onehunga Enhancement Society, which said the $800 million project would let Watercare discharge unlimited raw sewage across Manukau Harbour tidal flats, including the shores of Ambury Regional Park and Puketutu Island.
"Watercare has not done the analysis needed to understand the tidal flows in the upper harbour," said society chairman Jim Jackson.
Watercare Services replied that the structure's location was the best of the options investigated.
The Manukau Harbour Restoration Society and Mangere Bridge Residents & Ratepayers also objected.