$128m pipe saves the Bay

Photo / Jason Dorday

Officials have lifted the lid on one of New Zealand's greatest modern engineering feats - a 3km sewer under Auckland's Hobson Bay.

The tunnel, wide enough to drive a car through, will replace the surface pipe that has cut across the mouth of Hobson Bay for 90 years.

Testing on the $118 million system starts in October, and it's scheduled to be officially operating in December.

By then, work will have finished on a new pump station, capable of emptying an Olympic swimming pool in 8 minutes.

The six pumps are housed in one side of a shaft 35m deep - equivalent to a 12-storey building.

The other half is a "wet-well", a massive concrete chamber where excess wastewater can be stored during heavy rainfall.

Tunnelling took 10 months at a rate of 120m a week. A custom-built boring machine - itself 70m long - did the drilling and lined the tunnel with precast concrete segments at the same time.

More than 100,000 tonnes of rubble were removed on a small train rigged inside the pipe, which has now been removed.

At most points the 4.3m-wide tunnel lies 25m below the surface, and is slightly inclined so wastewater flows downhill from Parnell to the pump station.

The access shaft is slowly being filled as work winds up.

Project manager Mike Sheffield said the new pipe will almost eliminate more than 20 sewage overflows a year into the bay.

"There should be no overflow at all ... because the system has more capacity and more storage, but you can never say none," Sheffield said.

The project is being constructed by Watercare, which provides bulk water and wastewater services for the Auckland region. Once completed, the old concrete surface sewer will be demolished, opening up the area for watersports.

Sheffield said the concrete pipe will first have to be cleared of some 400 tonnes of debris.

"You can have anything from prams to tyres in these things because people throw all sorts of things into the stormwater system."