Recent heavy rain in the Tasman district has caused a large slip on State Highway 60 over the Tākaka Hill.
Waka Kotahi Top of the South system manager Rob Service said the slip was on the Riwaka side of the hill and temporary traffic management had been put in place.
There was a 30km/h speed limit and the highway had been reduced to one lane.
There would be delays of up to 10 minutes, Service said.
"It's important to acknowledge work to repair slips on State Highway 60 Tākaka Hill following Cyclone Gita is holding up well. This section of highway is not one that has required attention in the past."
An initial geotechnical assessment was carried out late last week, with further inspection work to be done.