Motorists are being asked to avoid State Highway 6 north of Nelson following a slip.
The slip was first reported about 10am today between Hira and Whangamoa.
Police confirmed the road was closed in both directions, until further notice.
A detour is available using SH63 via Renwick.
While the route is suitable for heavy vehicles, the Transport Agency said it would add about 75 minutes to travel time.
SH6 HIRA, NELSON - SLIP - 10:55AM— Top of the South (@NZTATotS) November 14, 2019
Due to a slip north of Nelson the road has been CLOSED between Hira and Whangamoa. The only detour available is via SH63, which will add close to an 90mins to your journey, so please take extra care. ^IF pic.twitter.com/upYRJGSEw8
Further south, SH6 is closed near Murchison after a boulder struck a car, killing a man yesterday afternoon.
The road is expected to remain closed throughout the day as rocks are removed from the slope above the highway.
The Transport Agency said motorists might need to delay their journey because there was no simple detour.
The highway is the main route between Nelson and Christchurch.
The best available detour for travellers is via Kaikōura and Blenheim using SH1 - this adds about seven hours to travel time.
A caution warning is also in place on SH6 between Kawatiri and Korere following a series of large landslips.