A critical final phase in the construction of Tauranga's $99 million Southern Pipeline has begun at Memorial Park.

Steel pipes are being thrust through a 1.6km hole drilled under the harbour to link Memorial Park with Matapihi Peninsula.

The work was taking place continuously, 24 hours a day, in order to keep the hole open and the pipes moving as they were winched and thrust, council project manager Steve Wiggill said.

The $21 million harbour crossing was the most technically challenging part of the project and was due to finish by the end of this year - seven years after construction of the pipeline began at Greerton in mid 2010.

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It will mark the end of waste bubbling out of manholes at the lowest point of central Tauranga's sewerage system when the Chapel St treatment works was unable to handle huge volumes of stormwater-swollen sewage.

Wiggill said the 24-hour work would continue for a minimum of four consecutive days and up to seven.

Noise reduction measures such as silencing generators have been undertaken to minimise the nuisance to residents living around the park. Wiggill said the noise would be carefully controlled.

In an update to residents, he said they might expect to hear occasional grinding, humming and limited excavator and crane movements.

''Closer to the site, you may hear some sound from the thrusting machine and mud pumps.''

The pipes are being lifted by cranes and positioned ready to be pushed into the hole.

They will also be pulled by steel rods from the drilling machine at the Matapihi side, he said.

Once the steel pipe was across, plastic pipes would be pushed through to ultimately carry the wastewater.

In an earlier update to residents, Wiggill said that if the work stopped, the pipe could get stuck.

The most technically challenging section of the project has lived up to its name, with the contractor encountering a series of problems, the latest of which was the discovery of much softer conditions than expected deep under the bed of the harbour.

"This has been an extremely challenging project. We've had to overcome several issues, beginning with an obstruction we encountered under the harbour back in August 2017 which badly damaged our machinery and created a domino effect of complications," Wiggill said.

A panel of global drilling experts was brought in to find a solution to stabilise the soft material.

He said the driver for the Southern Pipeline was the protection of the environment, and that continued to be the focus.

"We have been working closely with iwi representatives and meeting with residents close to the site to update them on the project. It's been a tough project but the team has overcome each issue they've encountered."


Southern Pipeline progress
- Original completion target early 2010.
- Laying of pipeline began mid 2010.
- Revised completion target December 2017.
- Latest completion target mid-November 2018.