Contract workers have been pulled from the Manawatu Gorge amid revelations a large slip could fall at any moment because of an unstable rock face.

New Zealand Transport Agency highway manager Ross I'Anson​ said on Friday engineers had confirmed a large area of rock face was highly unstable, with an imminent risk of further significant slips or rockfalls.

State Highway 3 through the gorge, between Woodville and Palmerston North, has been closed since April 24, when a large slip came down, covering both lanes.

SUPPLIED Geotechnical assessments have found a large section of the hillside through the Manawatu Gorge is unstable and could fall at any minute. Pictured is a slip that fell in the gorge in May.

"Geotechnical assessments have confirmed that the entire hillside is moving, and the rate of that movement is accelerating," I'Anson said.

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"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."

DAVID UNWIN/STUFF Transport Minister Simon Bridges tackling questions on the gorge route's uncertain future.

I'Anson said until the hillside at Kerry's Wall, 4.5 kilometres from the Ashhurst turnoff, stopped moving or slowed down it would not be possible to carry out further work at the site.

This meant they could also not predict when the road might reopen.

He said the news would add to the considerable frustration and stress local businesses and residents were already dealing with.

WARWICK SMITH/STUFF Woodville's main street has been quiet during the gorge closure.

"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."

He said transport agency was working closely with the Tararua District Council to do everything possible to help people and businesses in the region for as long as the gorge remained closed.

As the closure put considerable pressure on the Saddle Rd, the agency had agreed to take over full responsibility for its maintenance and management from the Tararua District Council.

DAVID UNWIN/STUFF No way through. The Manawatu Gorge road is now considered unsafe for repair works and will remain closed indefinitely.

This would remain for as long as the gorge was closed, with any repairs and remedial work funded by the agency..

I'Anson said urgent additional work would get under way immediately to significantly improve the Saddle Rd detour route.

"The significant length of the current closure has put the Saddle Rd under added pressure and the transport agency will be working to upgrade the road and keep it safe.

MURRAY WILSON/STUFF The first slip came down in April and the road has been closed ever since.

"The reality is that the Saddle Rd will effectively be functioning as the state highway connection for this part of the country for some time."

Roading officials had poured millions into improving Saddle Rd and I'Anson said more work was planned, but wet weather had delayed that work.

Pavement upgrades and repair work would start on sections of the Saddle Rd that hadn't yet been upgraded, on Friday.

The agency was also looking at a range of long-term options to bypass or replace the current Manawatu Gorge route, I'Anson said.

MAJOR BLOW

Palmerston North Mayor Grant Smith said it was fortunate nobody had been killed in the gorge to date.

The latest news had caught everyone, including the mayor, by total surprise, but he said said safety was a real concern.

He said the closure was an issue for the whole country, as the gorge connect east to west and was used extensively.

Any replacement would be a balancing act as residents of Woodville wanted the traffic, but residents in Ashhurst did not.

"We do want to bypass Ashhurst, it's just a matter of how we can do that."

Horizons Regional Council deputy chairman Paul Rieger said it was extremely disappointing to hear the road would remain closed as it was essential for heavy vehicles.

Having those vehicles using the Pahiatua Track and Saddle Rd was holding up massive amounts of domestic traffic, he said.

"It will be a major, major blow if we can't figure it out at a reasonable cost.

"[For] those who live around it and those who use it for commercial reasons, it's imperative we get a flat road open again."

Rieger visited the gorge a week and a half ago and saw a "massive amount" of water coming out of the hillside.

Workers were drilling about 50 metres into the hillside and inserting pipes, draining water out at an incredible rate, Rieger said.

At one point, he was told it was coming out at three litres a second.

"These slips don't happen in dry weather, it's water getting in."

Palmerston North MP Iain Lees-Galloway, of Labour, said the closure had an air of inevitability about it.

"Every year we have had major slips. The area is completely unstable and unsuitable as a safe transport route."

Lees-Galloway said he had no doubt the roads of national significance policy had prevented investment into building an alternative to the gorge.

"I think we are in a completely new place now and it would be appalling if the Government doesn't take action."

WOODVILLE'S WOES

Woodville has been particularly hard hit by the closure, as the traffic its businesses rely on has been diverted around the town.

Mayor Tracey Collis said she had been going around Woodville talking to business owners about the closure on Friday.

"Today was a hard piece of news and I guess it is a bit of a shock.

"We have been so close to opening and people have been holding out for that."

She said the hardest part for most people was not knowing when it would be open and how long they would have to manage for.

Signage had been ordered and was to be put in place by Monday, encouraging traffic to detour through Woodville.

Collis said businesses were meeting to discuss other options and needed to look at it as a long-term issue. She thought it would make Woodville stronger if it could get through this challenging period together.

Transport minister Simon Bridges sympathised with the frustration and stress Woodville and the other towns depending on the gorge road were feeling.

The Government had hoped to get the gorge open again as soon as possible, which would have solved Woodville's woes.

But with the indefinite closure, he was not ruling out some form of relief package for suffering businesses.

Bridges said the first, and best, plan on the table was for the transport agency to put up signs directing traffic towards Woodville from the Saddle Rd and Pahiatua Track routes.

There was a good chance that would return the town's traffic to nearly normal levels, Bridges said.

"In the event that's not working, and there's sustained economic losses in Woodville, we'll look at other options."

Bridges promised to keep a close eye on the situation. He also promised to visit the region's businesses and communities "soon" to discuss how their problems and what could be done while the gorge remained closed.

He said the Government was also "dusting off" options for a long-term fix.

"I can't hand on heart say what will happen, but I am committed to giving them another go [over]."

The options cost between $120m and $1.8 billion, and ranged from a re-route towards Saddle Rd and Te Apiti Wind Farm to a tunnel through the hills.

Those costs will be stacked up next to the gorge road's already high maintenance costs, the $20m in slip repairs over the past decade and the likely costs of future slips.

Bridges said they would also factor in the wider social and economic effects on the region each closure had.

"With 7000 vehicles going through it every day, the gorge is of vital importance to [the central North Island]."

Yummy Mummy's Cheesecakes owner Sera Williams said 80 per cent of the shop's revenue came from people travelling through Woodville.

They were already hurting after two months of slow business, and the longer the gorge was closed, the worse it will get, she said.

"We could survive [for awhile], but we'd have to lay off staff and that's not something we want to do. I just don't know what would happen next."

Williams hoped the Government would follow through on a long-term fix, as she wasn't too keen on the current stress all over again in a couple of years.

"It's a shame it's taken so long for them to think of coming up with something more permanent than just repairing it."

Rhys Punler, who owns a gift shop at The Bridge Cafe, said he was over the whole situation.

"I'm disappointed, but what can you do?"

Business had been on a downhill slide ever since the gorge closed in April and Punler said part of this issue was when people googled the cafe, Google Maps showed the road to the cafe and shops being part of the closures, when they were in fact open.

Due to the location of the alternative routes, Punler said they were not getting visitors from Hawke's Bay and Wellington stopping in on their travels.

He was not sure if he would stay in the shop, but he was trying to do what he could to support The Bridge Cafe.

"I went out there for $28 last weekend."

Woodville's DJ's Dairy Jitu Patel said business was very quiet.

"All the areas in town are dead.

"[Business] is maybe 30 to 50 per cent down, very down."

He said people were trying to think of solutions and he hoped the signage going up would help detour people through Woodville.

In a statement, senior constable Karl Williams said while absolutely necessary due to the safety concerns, police realise the continued closure of the Manawatu Gorge affected local businesses and caused extra travelling time.

"Police are urging residents and motorists to build extra time into their journey, enabling them to drive cautiously.

"If motorists don't plan extra time, they are placing themselves under additional stress while on the road, further increasing the risks to both themselves and others."