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A 400-metre stretch of State Highway 4 between Whanganui and Raetihi has completely collapsed after a slip, and there are concerns it could take years to rebuild.
Images show the broken Parapara Road looking as if it has been hit by an earthquake.
It means drivers will face detours of up to an hour to get between Raetihi and Whanganui.
National Road Carriers executive officer Tom Cloke drove to the site of the collapsed road on Friday and took photos of the scene.
He told Checkpoint it looked "like a bomb had gone off".
The landslip that has wiped out part of the Whanganui-Raetihi road. Photo: Supplied / Mark Brimblecombe Photography
"The slip has taken the road completely out, there's no way you can see where the road was or is.
"The road's gone into the river… It just looks like a hill with nothing on it."
Cracks were visible on the road on Monday, and by Wednesday the road was closed, Mr Cloke said.
An NZTA map of the collapsed Parapara Road. Photo: Supplied / NZTA
"Thursday it started to worsen and last night the road went completely. Thank goodness it happened fairly quickly and the road was closed because we could've lost somebody on the slope."
The fix was going to take a long time and Mr Cloke expected that will have a huge economic impact on the local economy.
"Not only that, it's the quickest route for medical emergency.
"I've seen a lot of slips in my time… and I think this is probably one of the worst I've seen."
Mr Cloke said the rebuild could be years.
"We won't be talking months."
The landslip that has wiped out part of the Whanganui-Raetihi road. Photo: Supplied / Mark Brimblecombe Photography