NZ Transport Agency Waka Kotahi (NZTA) said this month that the road is on track to reopen before Christmas, weather-permitting, and an exact date should be known in September.

However, heavy rain on June 19 and 20 threatened to further delay the opening with a massive slip falling down the hill.

“The recent weather last week … has caused further disruption to Mangamuka Gorge, with over 100mm of rain falling over a 12-hour period. During the heavy rainfall, a new slip formed just north of the northern-most critical slips,“ NZTA said.

“We don’t expect this new slip to affect the reopening date, and we’re still on track to be open by Christmas this year.“

NZTA said Mangamuka Gorge has its own unique challenges due to its geology and varying terrain.

A big issue is ‘Northland problematic rock’; it’s the main reason why SH1 over the gorge keeps slipping and has taken almost four years to fully repair.

Northland’s ground conditions (including through the Maungataniwha Range which surrounds Mangamuka Gorge) have always been fragile.

Almost 70 per cent of the geological material which forms rocks in the region is made up of Northland Allochthon – the ‘problematic rock’.

This type of material has long been challenging for construction and maintenance of roads, and means repairs to slips of this nature need to be carefully planned, since finding competent material to anchor the road to can be difficult.

The new slip is about 20m-long and 3m-deep from the edge of the road.

“As a precautionary measure, we closed the site on the afternoon of June 19, avoiding any night work during the stormy weather. The site has since reopened, and we’re monitoring ground movement.

“Geotechnical engineers are assessing the new slip to identify the extent of the damage and make a plan for its repair – we’ll give … more information once the engineers have done their work,“ NZTA said.

People can keep up to date with the Mangamuka Gorge work at nzta.govt.nz.