Construction of Tauranga's $99 million Southern Pipeline will end up being delayed by nearly another year after the contractor struck more problems trying to drill the pipeline under the bed of the harbour.

The most technically challenging section of the pipeline from Memorial Park to Matapihi Peninsula lived up to its name, with uncertainty around who would carry the cost overruns - the Tauranga City Council or the contractor.

Latest problems involved the discovery of much softer conditions than expected deep under the bed of Tauranga Harbour.

Council project manager Steve Wiggill said the soft material needed to be stabilised so the pipeline ran through firmer conditions. This plus the remaining work left to complete the project was expected to take another seven months.

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The $21m harbour crossing had been expected to be finished by December last year. The new timetable put completion out to mid-November this year.

Wiggill said the project budget was still $99m. It included a contingency - the size of which was commercially sensitive.

''There have been some cost overruns as a result of the delays, but it is yet to be decided who will carry these.''

The contractor planned to firm up the soft material by using harder drilling mud.

Wiggill said a smaller steel sleeve was being pushed through the 152cm outer steel sleeve driven into the bed of the harbour from the Memorial Park side.

As the smaller sleeve moved through the softer material into deeper, firmer material, a mixture of drilling mud would be injected to harden and stabilise the area of softer material, he said.

The contractor had so far finished more than two-thirds of the under-harbour reaming work to widen the initial 75cm diameter pilot hole drilled under 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 had been brought in to find a solution to stabilise the soft material.

Wiggill 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.''

After the soft material had been stabilised, work would resume to widen the final section of the harbour crossing. That was expected to take up to four weeks. Steel pipes would then be pulled through the harbour crossing, followed by the polyethylene wastewater pipes.

Wiggill said the final phase of this project was to connect the wastewater pipe to the previously constructed pipeline at Matapihi Rd and to the pump station in Memorial Park.

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