Motorists are advised to expect delays on State Highway One at Ohingaiti after a truck and trailer unit rolled there just before 3am on Tuesday.
The 20 tonne vehicle almost blocks both lanes.
Police say there is a significant buildup of traffic waiting to get through.
A crane is being brought from Palmerston North to right the vehicle but there will be delays until 8am at least. Another truck will take the freight away.
