KEY POINTS:
Roaring and spewing frothy black ooze like some angry subterranean monster, a giant machine burst into the daylight yesterday after grinding a 3km long sewer tunnel under the Auckland waterfront.
The breakthrough by the revolving cutting head of the 270 tonne machine came after eight months' round-the-clock work under the Orakei Ridge and Hobson Bay, starting from Orakei Domain.
Mayor John Banks, who was lowered 30m down a shaft in the bay to witness the last 700mm being bored, said the $118 million project was also an environmental breakthrough.
It would enable replacement later this year of the 90-year-old concrete pipeline that is an eyesore on Hobson Bay.
The machines are effectively giant drills which extract rock and then line the resulting tunnel with pre-cast concrete. Such machines were used to dig the Channel Tunnel between Britain and France.
Mr Banks said he was reminded this week of the need to rid the bay of "that antiquated structure" when an electricity substation at Penrose broke down causing the shutdown of the pumping station at Orakei Wharf which serves the other end of the tunnel sewer.
Raw sewage spilled from the station resulting in a public health warning to swimmers being posted at the beaches of Okahu Bay, Mission Bay, Kohimarama and St Heliers.
"It's simply not good enough causing public alarm on the eve of Waitangi Day whether they can walk on the beach," said Mr Banks.
The warning signs were removed yesterday.
Pollution by the pumping station would not have happened if the sewer tunnel had been in use, said Craig McIlroy, of Watercare Services.
"Under normal conditions, the new tunnel would give more than eight hours' capacity to store sewage, in the event of a power cut at Orakei pumping station."
On Tuesday, the old pipeline had only two hours' storage and Watercare brought in emergency generators to keep the pumping station working should there be rolling outages.
Mr McIlroy said work to commission the new tunnel would take place during November-December and the old pipeline would be demolished by July 2010.
On an average day the underground pipe, 3.75m in diameter, will carry about 3200cu m of sewage an hour. But during high rainfall, it will be capable of carrying 13,000cu m of sewage and water every hour.
Project manager Mike Sheffield told the Herald in November that the tunnel served two functions.
"It's not just about replacing the old pipeline, which is at the end of its economic life.
It's also providing significant storage capacity for high-flow periods, such as during a storm, which will virtually eliminate overflows into Hobson Bay."
Local iwi Ngati Whatua o Orakei call the tunnelling machine Te Kaha meaning "the strength". Kaumatua Danny Tumahai said its work was part of a marvellous engineering feat which would restore waters of the eastern bays to their former purity.
He looked forward to when Hobson Bay became a place for marine recreation.
Groups such as the Auckland Rowing Association see opportunities for rowing, kayaking, dragon boats, waka, small yachts, outriggers, Sea Scouts and swimming.