Watch: Growing concern as closed SH4 Parapara Rd between Whanganui and Raetihi deteriorates

Whanganui and Ruapehu residents are waiting with bated breath for updates after a geotechnical investigation began on Thursday following an underslip on State Highway 4.

And the region's leaders are already bracing for the economic impact.

The Parapara Rd was closed on Wednesday following an underslip approximately 500m north of the Matahiwi Track. Photo / Supplied

The Parapara Rd between Whanganui and Raetihi has been closed since Wednesday after a huge crack appeared about 500m north of the Matahiwi Track.

Since then an entire hill face has slipped taking a huge section of road with it.

By Friday, the piece of road concerning contractors on SH4 had completely fallen away after being described as an "unstable" on Wednesday. Photo / Supplied

Local farmer Rex Martin said the road would take months to fix and executive officer of the National Road Carriers Association Tom Cloke agreed it would be a long-term closure.

Whanganui & Partners chief executive officer Mark Ward said if SH4 is closed for a significant time, it would have a major impact on Whanganui's visitor economy.

"It will have a major impact on the international visitor segment. That's 20 per cent of our visitor spend which is about $24 million per year," Ward said.

"We could also lose a significant percentage of the domestic visitors, the other 80 per cent of our visitor spend. Our total visitor spend is $132 million."

Whanganui mayor Hamish McDouall said it is more of a problem for locals.

By Friday, the piece of road concerning contractors on SH4 had completely fallen away after being described as an "unstable" on Wednesday. Photo / Supplied

Local farmer Rex Martin took this picture on Friday morning with the caption "update: road gone". Photo / Supplied

"To be frank, the percentage of tourists who come down the Parapara is actually remarkably small, it's less than 10 per cent of the tourist traffic," McDouall said.

"Most of the tourists use State Highway 3, but really the problem is for locals, people like ourselves who might want to travel. It adds significant time to our journey."

Before now, the longest time McDouall could remember the road being closed was three weeks following the Whanganui floods in 2015.

He said a potential long-term closure would also affect freight and other things.

"You've got to think about emergency services as well," McDouall said.

"It does beget a bigger question about realignment of the Parapara. I know NZTA had started a discussion about some significant areas that were of concern."

The Ruapehu region will also be affected by the closure as tourists may be deterred from skiing at Mt Ruapehu and emergency services will have a longer route to Raetihi.

Ruapehu mayor Don Cameron said what was happening with the highway is very concerning.

"The crack is huge and from the photo I saw taken on Wednesday morning and looking at it now, it has shifted quite dramatically," Cameron said.

"For Whanganui, a lot of goods are delivered down that road and it will be extremely expensive for trucking companies to take another route."

The Ruapehu District Council (RDC) is in the process of investigating what economic impact the road closure could have on the region.

If SH4 closes for a significant period it would affect even more motorists trying to enter Ruapehu as the SH43 Forgotten World Highway is already down to one lane.

Cameron said they want to make a business case for a complete upgrade of the Parapara Rd and SH4.

"Unless they can get some agreements with land owners to shift the road across the other side of the Mangawhero River, we're forever going to have problems.

"It's unfortunate, they were about to start a whole road upgrade this summer and now they've got this to contend with, which looks to be really huge."

Cameron said RDC chief executive Clive Manley spoke with NZTA Regional Relationships Lower North Island Director Emma Speight on Thursday morning.

Manley called Cameron after the discussion, telling him Speight confirmed a major upgrade to the Parapara Rd due to commence this summer would still go ahead.

While the road is closed, the detour routes for motorists in light vehicles is through SH1 and Fields Track, while heavy vehicles are advised to use SH1 and 49.