SH1 has been closed for repairs since February, with another landslip hitting just before it was due to re-open. Photo: Supplied/ NZTA
Soil is continuing to move at the top of the slopes above a big landslip on State Highway One over the Brynderwyns.
The slip hit a few days ago, just two weeks before the road near Whangārei was due to reopen.
State Highway 1 through the Brynderwyn Hills was damaged in last year's Cyclone Gabrielle, and had been closed for repairs since mid-February, with the road reopening temporarily over Easter.
NZTA Waka Kotahi said it expected to resume earthworks at the top of the slope this week, if geotech experts said it was safe.
"At this point it's too early to know how this will impact on our overall costs," project director Mark Ware said in a statement.
There were already going to be more earthworks done in this area as part of the design, with the slope due for "substantial" earthworks.
"The impact will be determined over the next few weeks as we understand volumes and the geology of the slopes further."
A safety review was being carried out by the contractor, with the transport agency also bringing in an independent geotech engineer to look at the design and approach.
"As a matter of course, our contractors undertake a review to see if there are lessons to be learned," Ware said.
RNZ has requested both those reports when completed.
Workers had early warning of the movement, so cordoned the affected area off.
"Site monitors and geotechnical specialists had identified early ground movement," Ware said.
A small slip on 26 April was followed by a bigger one four days later.
"We also have ground sensors throughout the area that provide early warning. This equipment is in place because we know that this area is susceptible to slips while the stabilisation work is ongoing."
About 45,000-55,000 cubic metres of soil had to be removed from the new slip, on top of the 180,000 cubic metres already moved.
"Work on slopes that require a full closure will be completed before the road reopens, ensuring the slopes will be safe for all road users," Ware said.
Some ongoing stabilising and pavement works could be done later, using the newly created road-shoulder space.