Northland storm: Slip that closed Mangamuka Gorge could take 6 weeks to clear

A large under-slip has undermined State Highway 1 in the Mangamuka ranges south of Kaitaia with the transport agency unable to say when the road will reopen.

A large slip below State Highway One at the Mangamuka Gorge has left roading authorities unsure when the road through the gorge will reopen, but it could take six weeks.

A series of slips closed SH1 through Mangamuka Gorge overnight on July 17/18 when a storm dumped 220mm of rain - about two months worth - in a few hours.

Waka Kotahi NZ Transport Agency says it's still too early to say when SH1 through the gorge could be opened to traffic.

Most of the road has been cleared to at least one lane, but there's still no way past the biggest slip on the northern side of the gorge. The hillside under the road fell away during the heavy rain two weeks ago, leaving metres of road surface suspended in the air.

Waka Kotahi Northland System Manager Jacqui Hori-Hoult said safety was a priority as her team assessed the slip and the repair job needed to reopen the road.

"Six to seven thousand cubic metres of material has fallen from under the road to the river below. What's left is brittle and unstable and our concern is that more heavy rain could bring down more of the hillside,'' Hori-Hoult said.

"A further complication is that there's a historic slip directly above the road and we have to be very careful about disturbing the slope. We need to further investigate the slope to develop our longer-term road repair options.

"The good news is that there's been only light rain since the slip. Our geotech experts are working to understand the soil conditions and locate stable bedrock before deciding on the safest and most effective solution for restoring the road."

The plan is to work in stages to open the road, initially to a single lane so that light vehicles can get past. Weather permitting, that may take six weeks.

Elsewhere in the gorge, other slips have been cleared and, with safety barriers and other precautions in place, the road is ready for light vehicles.

A large under-slip has undermined State Highway 1 in the Mangamuka ranges south of Kaitaia with the transport agency unable to say when the road will reopen.

"Waka Kotahi acknowledges the inconvenience and cost to local communities on either side of the gorge of having to use the longer SH10 detour. We're working to reopen the road as safely and as quickly as we can," she said.

On average, about 1300 vehicles a day travel on SH1 through the Mangamuka Gorge. It will be some time before trucks and buses can travel the road.

For now, SH1 through the Mangamuka Gorge is closed between Victoria Valley Rd and Makene Rd. SH10 is the detour with motorists advised to allow an extra 20-30 minutes for their journey.

The estimated cost of the storm response and repairs to Northland's state highway network to date is $5.9 million. This did not include the final repair costs for the Mangamuka Gorge, Hori-Hoult said.