The Manawatu Gorge is closed by a slip, the second time the main route south from Hawke's Bay has been closed within three days.
The slip, which could keep the key east-west Hawke's Bay-to-Manawatu link closed for three weeks, came after another slip hit at least two cars and closed the Rimutaka Hill section of State Highway 2 between Wairarapa and Wellington for several hours at the weekend.
NZ Transport Agency highway manager Ross l'Anson said there were two slips, one of about 200 cubic metres at the southern end - expected to take about a day and a half to clear - and a much larger slip of about 3500cu m towards the Woodville end.
It could be three weeks before the larger slip was cleared, he said.
"Geotechnical engineers and Transport Agency staff have done a preliminary assessment of the slips with a drone flyover, ensuring that it is safe for crews to start clearing slip material and stabilising the slopes," he said. "More detailed analysis will now be done and a further meeting between the NZTA and engineers today will decide on the next steps."
Motorists are being advised to use either the Pahiatua Track to the south or the Saddle Rd to the north as alternative routes across the divide, or the Wairarapa route to Wellington.
The gorge has been closed regularly over recent years, most significantly for 16 months after a 140m-wide slip in August 2011, which contractors said was the largest road landslide in New Zealand history.
It required the removal of rock and other debris said to have been enough to fill stadiums the size of Napier's McLean Park to roof level.
A remedial project costing more than $2 million was later begun, including mesh drape over rock faces.