A fresh slip has ripped across the Manawatu Gorge, blocking an area just cleared after several weeks work.
The New Zealand Transport Agency said the slip has further blocked State Highway 3, in the same location as contractors had been working.
Torrential rainfall over the weekend further destabilised the cleared site and approximately 20,000 cubic metres of new material came down last night, said NZTA regional state highways manager David McGonigal.
The slip blocked the section of road that had been finally cleared on Friday after weeks of work due to previous slips.
Mr McGonigal said NZTA contractors would begin clearing the new debris as soon as it was safe to do so, and further material was expected to come down, particularly following rainfall.
It was too early to say when the road would be reopened and assessments could not be done until the conditions were safe.
"We appreciate that people are frustrated about the length of this closure, and so are we. We know how important this road is for local communities and for the region's economy, and we are working hard to get it re-opened as soon as possible.
But before we can allow traffic to use this route we must ensure that the site is stable and the road is safe to travel. The scale of this latest slip shows that isn't yet the case.
"This is one of the longest and most frustrating closures on record - we know how disruptive it has been for motorists and local communities, and we thank them again for their patience."
Detours are in place which the NZTA estimates adds about 15-20 minutes to a journey.
The gorge has been closed for several months, threatening the viability of businesses reliant on through-traffic.
The Manawatu Gorge's four-month closure may be the "final blow" for businesses that rely on its traffic, the region's economic development agency says. The longest previous closure was for 70 days in 2004.
The NZTA took over control of detours through the Saddle Road and Pahiatua Track earlier this month after receiving geological assessments which indicated the closure was likely to continue for up to two months more.
"Closures of this extent only happen around once in a decade, but when they do happen they cause real disruption. Our priority is twofold: getting the highway reopened as soon as it's safe to do so, and keeping the alternative routes well maintained and safe in the meantime."
The NZTA advises motorists that the detour routes take on average 15-20 minutes longer than SH3 through the gorge.
- The Dominion Post