State Highway 4 road reconstruction estimated to cost more than $20m

NZTA workers drilling boreholes and installing groundwater monitoring devices at Te Oreore on State Highway 4. Photo / Supplied

The total cost of repairing the Te Oreore slip area and permanently reinstating State Highway 4 between Whanganui and Raetihi is estimated to cost more than $20 million.

Construction of the new stretch of highway would begin in "summer 2021".

The NZ Transport Agency says $5.2m has been spent on the project to date and estimates a further $15.3m will be needed for the permanent reinstatement of State Highway 4.

NZTA Regional Transport Systems Manager Ross I'Anson said deep extraction wells and pumps were set to remove groundwater from the slip area into newly upgraded drains and take water away from the site.

"This followed preparatory work which included creating access tracks, upgrading the drainage, drilling boreholes, taking samples deep below the road and putting in monitoring equipment," I'Anson said.

"We are monitoring any land movement to check the impact of changes in groundwater levels or earthquakes."

The 2019 slip was caused by water gradually building up in the soil and rock from groundwater seepage, I'Anson said, and that in turn increased the water pressure at the base of the landslip, reducing the soil's ability to resist ground movement to a point that the landslip was initiated.

"We would like to thank landowner Ātihau-Whanganui and mana whenua Ngāti Rangi for their generosity and support throughout the process of building the temporary road and looking at options for the permanent route."

Ruapehu District mayor Don Cameron. Photo / Bevan Conley

Ruapehu District mayor Don Cameron said the council met with NZTA last week, and that the repairs "weren't going to stop".

"There was a little bit of a hold up, mainly around obtaining landowner consent and buying land," Cameron said.

"They (NZTA) have to make it resilient because the closure of State Highway 4 would just be chaotic."

"The locals are really thankful for what's going on, and every day it gets a little bit faster to get to Whanganui."

Cameron said Raetihi was "thriving at the moment", and that the Te Oreore slip was being repaired in "record time".

"NZTA is factoring climate change into their repairs and maintenance now, and the region has actually sent a letter to them to say thanks for the way they've responded to State Highway 4 and the Manawatu Gorge as well."