A slip has closed State Highway 25A in the Coromandel in both directions.
The road, which connects Kopu, near Thames and Tairua is the main link between the east and west side of the Peninsula.
Police said the slip, near the summit of the road, was about 50 metres wide and blocking both lanes.
It is expected to take until morning to clear.
Motorists will need to take lengthy diversions to the north or south in the meantime.