Te Mārua Water Treatment Plant. Photo: Supplied
Building costs are increasing due to tougher design standards to help them withstand earthquakes.
Seismic risk guidelines have been tightening since the 2016 Kaikōura quake - and are rising again.
In one instance, changes since 2022 had added millions to a $90 million water treatment project in Wellington.
Greater Wellington Regional Council confirmed to RNZ that cost increases related to addressing seismic risk were around $5m due to higher design and construction prices, including for the building of a large water-retaining structure at the Te Mārua water treatment plant.
This comes as the government consults on a higher national standard for seismic design, with the aim it eventually becomes mandatory.
Commercial property lawyer Doran Wyatt said early estimates showed the tighter standards could add 5 to 7 percent to new building costs.
Central to the issue is the national seismic hazard model, which was updated 18 months ago.
Following this, assessed hazard more than doubled in some areas, including in parts of Auckland, Wellington and Christchurch. It indicated that worse shaking could be expected in a quake than previously thought.
In response, a committee of engineers came up with a new design standard, moving it closer to those in other areas of high seismicity, such as Japan and California, Wyatt said in a post.
While it was only a guideline for now, the standard was on track to be made mandatory, and "it would be sensible to take [it] into account now in the design of new buildings", he said, adding: "A failure to do so could well mean a new building that underperforms."
The cost blow-out for the Wellington water treatment project - which is right next to the Wellington Fault - was due to an earlier design revision.
"The guidance was updated in 2022 and highlights that seismic risks are higher than previously understood," the council said in a statement.