One of the country's most expensive stretches of new highway just got more costly - and further delayed - by buried pumice that wasn't spotted despite scores of ground tests.

The 2km stretch of the Bayfair to Baypark upgrade on State Highway 2 was already costing $70 million per kilometre - two to three times more than usual.

But three years after construction began in 2017, the New Zealand Transport Agency Waka Kotahi has realised the ground is not stable enough for what it is building, after a pumice layer was uncovered during work.

That forced an ongoing, major redesign and related changes to construction methodology and to the rest of the Bay Link programme.

NZTA says new equipment must be brought in, under a new contract, to begin building new flyover foundations, probably in November.

Tauranga Mayor Tenby Powell is surprised and frustrated.

He was not aware of the issue until RNZ told him on Wednesday.

"That is very frustrating for New Zealand's fastest-growing city."

100 tests - no pumice

More than 100 ground tests were done before construction began, says NZTA.

But they failed to spot the pumice layer 12m underground, so groundworks carried on.

Tauranga is on the edge of the Taupō volcanic zone and Bay of Plenty has a lot of pumice underground.

The fact that groundworks failed to properly stabilise the ground only came to light after huge stone columns began going in 10 months ago.

That sparked more tests, including earlier this year at Auckland and Canterbury universities, and the scrapping of the original design.

NZTA says a new design was being finalised.

It was still working out the impacts on the project's cost and timing.

NZTA did not respond to an RNZ question about whether extra costs would fall on the taxpayer.

Even before the pumice problem forced the redesign the project costs had escalated from an early $102m estimate to $146m recently - and its completion had been pushed out from end of this year to end of 2022.

Other complications are the fallout from COVID-19 and an underpass being added to the project.

The B2B project, as it is called, aims to cut congestion on the final run into Tauranga at Baypark where the tolled Eastern Link highway stops - and the traffic slows to a crawl.

The flyovers will take traffic on State Highways 2 and 29 up over roads and the railway line.

Tauranga Mayor Tenby Powell.

The highway is also aimed to boost the local economy and safety for motorists.

Powell says it's vital to complete the region's Eastern Corridor.

"We've got congestion on many of our routes into and out of the city, and we have a tonne of work ahead of us for years to come, so that is very frustrating to learn," he says.

NZTA has not explained how the pumice was not detected at the start.

"This type of testing is not unusual," it says.

"Projects typically do continual tests of ground conditions as they progress through construction and this process can sometimes identify previously unknown ground conditions."

The new design - a combination of stone columns and piles in a lattice framework - was "required to limit settlement and ensure the ground remains stable, ensuring the bridge [at Baypark] is able to withstand seismic events".

The B2B is the latest problem project for the NZTA.

It is grappling with blowouts at the billion-dollar Transmission Gully highway, costly surface cracking on the Kapiti and Waikato highways, and extra engineering reviews and deck-panel testing at the Puhoi to Wellsford highway.

The main contractor at B2B, Australian major CPB, declined to comment. CPB is also key in the Transmission Gully alliance and lead contractor at the strife-hit Acute hospital project in Christchurch.

Powell says NZTA had been good to work with, but had not treated the council as a partner by not telling him about the B2B problems.

"I'm gonna pick up the phone and call the regional director and just try and get an understanding of what exactly has happened, and particularly why we aren't on the same songsheet in terms of communications."

-RNZ/Phil Pennington.