The route through the Manawatu Gorge has been cleared by geotechnical engineers after a large slip closed the road for a day.
Heavy rain caused 200cu m of rock and debris to fall, about 1.30am on Friday.
The New Zealand Transport Agency closed State Highway 3 through the gorge following the rockfall, nearly 5km northwest of Woodville. Traffic was redirected to the Saddle Rd.
Geotechnical engineers examined the hillside using a camera-equipped drone and checked material above the slip to ensure it was safe for work crews to begin shifting material away from the road.
"There [was] further unstable material, which will also need to be removed," a NZTA spokesman said.
Major work was still being carried out late yesterday, with contractors sweeping debris from the road about 6pm.
Last week, a truck carrying animal blood blocked a section of road after it went through a barrier and ended up hanging precariously over the edge.
Police initially closed the gorge at both ends, before opening one lane.
Traffic was also disrupted last month as minor slips and safety checks were done on areas that could pose slip risks.