The Manawatu Gorge is likely to be remain closed for some time due to fresh fears about another major slip.
The slip in the Manawatu Gorge. Photo: NZTA
The Transport Agency said a large area above the Kerry's Wall rock face was highly unstable, and it had removed all contractors from working there.
"Geotechnical assessments have confirmed that the entire hillside is moving, and the rate of that movement is accelerating," the agency's Ross I'Anson said.
"That's an indication that a slip as large or larger than the 2011 slip which closed the road for 14 months could come down at any time."
State Highway 3 through the gorge has been closed since April with traffic diverted over the Saddle Road.
Mr I'Anson said until the movement of the hillside at Kerry's wall slowed or stopped it would not be possible to carry out further work at the site or predict when the road might re-open.
"We know that this news will add to the considerable frustration and stress which local businesses and residents are already dealing with," Mr l'Anson said.
"We understand the impact that this ongoing closure has on people's lives, but the safety of road users is paramount and the current unstable conditions in the gorge mean it simply cannot be opened to traffic until more work is done to fully understand the risks and how they might be mitigated."
The gorge road has been closed since April and Mr I'Anson said its re-opening had been deferred several times, due to more problems.
"We've been ready to open the gorge four times in the last two months and every time we've been about to open something else has happened.
"Over the last month this monitoring has shown us that there is a much greater risk there than we knew about, we had been monitoring it for some and it was going very, very slowly, to no movement at all to suddenly there's a large movement."
The Transport Agency said it was already investing $8.5 million in an upgrade to improve the Saddle Road to cope with increased traffic volumes.
Mr I'Anson said more work was planned.
"The significant length of the current closure has put the Saddle Road under added pressure, and the Transport Agency will be working to upgrade the road and keep it safe," he said.
"The reality is that the Saddle Road will effectively be functioning as the state highway connection for this part of the country for some time."
NZTA will look at a range of long term options to bypass or replace the Manawatu Gorge route.
Mr I'Anson said that had to be an option because there had been issues in the Manawatu Gorge for the last 100 years and that would continue due to the gorge's geology.
He said a report in 2012 looked at four options for the Manawatu Gorge at a cost of $120 million to $1.8 billion.
The alternate route over Saddle Road bypasses Woodville, and some businesses there say they are suffering as a result.
Mr l'Anson said the Transport Agency will install extra signage pointing to Woodville.