Erosion caused by rising river levels has forced transport authorities to close the route through the Manawatu Gorge and warn that it may not reopen for six weeks.
State Highway 3 was closed late on Thursday night after damage to the temporary road made it unsafe a little more than a month after it opened.
The road through the gorge, linking Manawatu with Hawke's Bay, was closed for more than nine months after a series of slips dumped 300,000 cubic metres of rocks and dirt on the highway in August last year. It partially reopened on May 31.
The Manawatu River was running at levels almost seven metres higher than normal, and had eroded some of the slip material that supported the road.
"The road was closed before anyone was put at risk," said New Zealand Transport Authority Palmerston North state highways manager David McGonigal.
He says the authority was aware of the problems that flooding could cause but despite moving the road a further five metres in from the river's edge this week, increasing the buffer zone to 10m, it was still not enough.
"The intensity of the flooding in the river has simply eaten away so much of the slope that we've lost a good chunk of the temporary road as well."
Mr McGonigal says the damage meant it was unlikely the temporary road would be rebuilt and it would instead work to complete bridges destroyed in last year's slip.
The agency would know by the middle of next week how long work on the bridges would take to compete, but Mr McGonigal warned estimates put it at between four to six weeks before the road reopened.
Motorists were advised to use the Saddle Road or Pahiatua Track.