Multiple slips will keep the Manawatu Gorge closed for three weeks but it remains unclear what caused them, the Transport Agency says.
Contractors have been assessing the damage and working to clear the slips on State Highway 3 between Ashhurst and Woodville, which is covering both lanes.
The Transport Agency said drivers should use Saddle Road or the Pahiatua Track as alternative routes while the gorge was closed.
Its highway manager for Palmerston North, Ross I'Anson, said the slips might have been caused by seismic activity near Hawke's Bay last night, or just by the build up of moisture in the hillside.
Mr I'Anson said the slip at the Ashhurst end was about 200 to 300 cubic metres, and was only expected to take a day and a half to clear.
The slip at the Woodville end was much bigger, about 3000 cubic metres, and was sitting on top of a bridge, he said.
The structure of the bridge would also need to be checked once the slip was cleared.
A drone and a geotechinical engineer had gone into the area today to assess the stability of the ground above the slips, and work to clear the slips would start tomorrow.
There was on-going work underway to try and strengthen the road long term and Mr I'Anson said there would be about $850,000 spent on the road this year.
He said both areas had netting on them but the slips were large enough to break through it.
Manawatu Gorge is prone to slips and was closed for over a year in 2011 and 2012 after massive landslips pushed 375,000 cubic metres of debris over the road and damaged several bridges.
More recently, smaller slips closed the road for a month in 2015.
The Transport Agency spent about $10 million on a major upgrade of Saddle Road in 2015 to make it a more viable alternative route.
Part of SH36 in the Bay of Plenty would also stay closed today, as contractors worked to remove an overturned truck carrying hazardous materials.
A fatal accident occurred between Pyes Pa Road and Te Matai Road, near Rotorua yesterday.