State Highway 6 is closed between Nelson and Blenheim after wild weather downed trees and caused slips on Monday night.
"Bad weather has brought down trees and caused rockfalls on the Whangamoa Hill. It means State Highway 6 will be closed between Rai Valley and Hira overnight," Waka Kotahi said in a statement.
"Gale force winds and darkness mean it is too dangerous for contractors to clear the road tonight.
"It may take some time tomorrow for the road to be cleared."
An update will be provided by midday on Tuesday.
"The closure means there is no direct route between Nelson and Blenheim. Road users should delay non-essential travel or use the only alternative route."
Motorists travelling from Blenheim have been advised to travel via SH63, Wairau Valley to Kawatiri Junction and SH6 to Nelson.
"This is a significant detour and will mean much longer travel times."
Motorists have been urged to exercise "exercise extreme caution on the roads".
Traffic management will be in place.
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