Work will start in January on a $91 million tunnel to take treated effluent across North Shore City and discharge it into the Rangitoto Channel.
The tunnel will be the single most expensive project the city has undertaken and has survived proposed council spending cuts.
It will pass25m under city streets, parks and homes for 2.6km from the Rosedale Treatment Plant to Mairangi Bay.
Council wastewater networks manager Dave Woods said the tunnel would take "high-quality" treated effluent from the plant to a point offshore and discharge it 2.8km out to sea in a section of pipe buried in the sea bed. It was part of the city's $250 million Project Care to reduce pollution on beaches over 20 years.
Mr Woods said a new tunnel was needed to meet the growth of the city for the next century. It has six times the capacity of the present tunnel, which is 40 years old and has needed major repairs.
The present outfall discharges only 600m off the coastal Kennedy Park and has been the subject of complaints over the years because of discoloured water and streams of bubbles.
Its replacement is designed to avoid causing this by diffusing the treated effluent over a wider area.
The Auckland Regional Council has granted variations to resource consents gained in 2001 to allow different construction methods that will reduce the risk of tunnelling causing ground settlement.
The ARC has imposed a deadline of late 2010 for completing the works.
The successful tenderer, McConnell Dowell Constructors, will start physical works in January to meet that deadline.
Increasing tunnelling costs have concerned the council, which in 2004 received an estimate of $75 million.
The proposed route has since been simplified to save $7 million but councillors believe it will still cost about $95 million.
Mayor Andrew Williams confirmed that the project would go ahead despite his cost-cutting drive, saying the deal was signed by the previous council and was in the budget.
