A truck and trailer unit rolled on State Highway 1 south of Waiouru early on Monday, blocking the road.
Police say the driver who was initially reported to be trapped in the vehicle, was freed and taken to Whanganui Hospital by ambulance.
The road was blocked in both directions while equipment was brought in to clear the scene. Diversions were in place.
Police said it was likely to take several hours to remove the truck and trailer and for the road to reopen.