State Highway 6 between Korere and Glenhope, south of Nelson, remains closed by a series of large landslips.
It was closed at 7.30pm yesterday and police said it would be some time before debris was cleared along the 40km stretch of road.
Slips, flooding and fallen trees were reported in the area.
Traffic is being diverted through St Arnaud.
The New Zealand Transport Agency said the slips closed a section of the highway yesterday and were still being assessed by a contractor.
A spokesperson said a detour was in place at the State Highway 6/Kikiwa turnoff, for those heading south, and Kawatiri - SH63 for those heading north.
The detour was said to be suitable for heavy vehicles.
NZTA said it hoped for an update this afternoon as to when the road might re-open to one lane.
Motorists were earlier asked to delay their travels or take a detour on State Highway 63.
Fire and Emergency responded to 15 weather-related calls yesterday evening across Northland, Auckland and Waikato.