State Highway One's closed between Ashburton and Rangitata, after a truck and trailer unit rolled this morning.
Police say the crash happened at Tinwald in Ashburton about 5.30am on Thursday, near the intersection of Hendersons Road.
Detours are in place, with northbound traffic using Winslow Willowby Road, and southbound traffic using Longbeach Road.
"The truck is clear of the railway lines, however the road is completely blocked," police said.
Diversions are in place but drivers are being warned it could take some time to clear the scene.
No one was hurt and the driver already out of the overturned cab by the time fire crews arrived.