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 chief executive 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 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.