A truck has rolled between Springs Junction and Murchison in the Buller District.
State Highway 65 is down to one lane as cranes remove the wreckage.
Police said no-one was injured in the 4am crash and while the scene is being cleared, the highway will be down to one lane with stop/go signs used to control traffic.
The road is not expected to fully reopen before 5pm.
Police thanked drivers for their patience and advised allowing extra time for their journeys.