Increased construction costs and a higher standard of earthquake proofing has driven up the cost of Whangarei's Hundertwasser Art Centre by $4.7 million, bringing the total cost of the project to $20.9m.

Revised costings released by Prosper Northland Trust (PNT) today, show an additional $3.6m in engineering costs for seismic strengthening, as well as $1.1m in increased construction costs.

PNT chairman Barry Trass said although the Whangarei District Council (WDC), which owns the Town Basin land where the centre will be built, had done engineering reports on the site, PNT wanted to future-proof the public building with structural improvements.

As part of its due diligence process, PNT commissioned engineers to conduct soil testing onsite last September and had engineers review the findings.

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The impact of the Kaikoura earthquake on Wellington further boosted the trust's motivation to ensure the Whangarei building could withstand seismic activity, in the rare chance that it could occur.

The stability of the land around the former Whangarei Harbour Board buildings was tested by WDC staff in 2012, following sweeping changes to building legislation after the Christchurch earthquake.

"To be fair to council - things have changed dramatically around earthquakes in the last two years," said Mr Trass.

Earlier plans were no longer considered adequate for the project under revised engineering standards, he said.

"We are being prudent . . . we want the building to stand the test of time," he said.

The trust had to revise the Hundertwasser Art Centre and Wairau Maori Art Gallery plan regarding seismic strengthening and a beefed-up foundation.

Mr Trass said those factors were outside PNT's control but the trust was satisfied the plan now met a much higher structural standard.

"We are comfortable with the revised costs, and the resulting plans which ensure the Hundertwasser Art Centre with Wairau Maori Art Gallery is a lasting asset for our community," he said.

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The costing review also tallied up rising construction costs, with industry prices rising since initial costings in 2015.

Under the WDC referendum conditions, PNT had until June 30 to raise $1.6m of the $16.25m construction costs, as well as the 10-year $2m underwrite.

It has a further year to raise the additional $4.7m and Mr Trass was confident funds would be found well before that.

PNT had to wait until the project funding was about 80 per cent through before investing tens of thousands in the core sampling onsite.

As well as rising construction materials and labour costs, the building of Te Kakano had taken builders three times longer to finish than the average building, because of the intricate detail.