KEY POINTS:
The waters off the North Shore are again the scene of action and drama - as they were in the America's Cup yacht races a few years ago.
This time, the hand-picked crews are wearing hard hats and overalls or diving suits.
Their vessels are tough-skinned beamy barges, stout enough to hold a bucking 75-tonne excavator as its brawny steel arm reaches into the Rangitoto Channel to gouge a trench in the rock and mud.
They are working in shifts to construct a 2.8km buried pipeline out from the Mairangi Bay reserve and under the beach and seabed to where the water is 12m deep.
Sticking out of the last 350m of the pipeline will be 58 outlets called diffusers which will disperse treated effluent into the strong ocean currents.
Looking like giant golf tees, the stiff rubber hose diffusers can withstand a wallop, too. They will bend if a boat's anchors or ropes snag on them.
A large rusty brown pipe has protruded from the bottom of the sea off Mairangi Bay since before the Christmas holiday. Called a riser, it marks the landward end of the submarine pipeline.
Here, the pipeline will be connected to a tunnel which goes 2.6km to North Shore City Council's Rosedale Water Treatment Plant.
Far below houses and parks on the way, a boring machine the size of a freight truck is chewing through rock around the clock, the tunnellers steering a curving route to meet the riser.
McConnell Dowell is contractor for North Shore City Council's $116 million outfall project.
Company project manager Greg Wichman and Kristian Nelson, who manages the marine part, said the pipe would be buried in a 3.3m-deep trench and covered over with at least 1m of seabed material.
"The object is to get the treated wastewater up from the level of the tunnel and convey it to the diffuser with a minimum impact to the environment," said Mr Wichman.
"The only items visible on the seabed will be the access ports and the risers."
The marine pipeline is 1.6m in diameter and 65mm thick. It is made in Thailand of high-density polyethylene. "It's in the same family as the material used in shopping bags".
The 12m lengths of the pipe are shipped to Kaiaua on the Firth of Thames where they are being welded into strings that will float.
Like links of black sausages, the pipe strings will be towed by tugboat to a huge yellow barge moored off Mairangi Bay, which is modified for a work platform and has a 120- tonne crawler crane for some lifting muscle.
Barge workers will clamp 10-tonne concrete weights to the pipe strings, which are pulled off the barge and lowered into the trench by controlled flooding and sinking of the string.
Once the strings are firmly settled on the sea bed, divers will insert a short length of pipe between the ends and bolt them into place to form a continuous pipeline.
After pressure testing for leaky joints, the pipeline will be buried by the dredging barge's excavator, using material that was scooped out for the trench and heaped beside it.
The council says the outfall will have a lifespan of 100 years and gives the treatment plant six times more capacity than the present outfall, which discharges 600m off Red Bluff, J.F. Kennedy Park.
* Key dates
February: Laying of pipeline in the trench begins.
April/May: Dredging a 2.8km trench in the seabed completed.
September: Tunnel pipe reaches Mairangi Bay beach.
November: Tunnel pipe connects to "riser" off the beach.
Late next year: Commissioning of new outfall.
