Manawatu Gorge will be closed for up to three weeks while a large slip is cleared.
There are two slips blocking both lanes on State Highway 3 between Ashhurst and Woodville.
The larger slip of approximately 3500 cubic metres is at the north-eastern (Woodville) end, and it could take up to three weeks to clear before the road is safe again, NZTA said.
In the meantime motorists can use the Pahiatua Track and the Saddle Rd as alternative routes.
The smaller slip, of about 200 cubic metres, is at the southern (Ashhurst) end of the gorge and will take about a day and a half to clear.
While it may seem large, the northern slip is small compared to a huge 2011 slip which closed the road for 16 months, while contractors removed about 370,000 cubic metres of soil, rock and other debris.
Businesses impacted after the gorge was closed in 2011 due to the slip say they're nervous they could again lose out.
"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," an NZTA spokesperson said.
"More detailed analysis will now be done and a further meeting between the Transport Agency and engineers [Wednesday] at 9am will decide on the next steps."