One person is believed to be critically injured after a truck collided with another vehicle south of Palmerston.
A police spokeswoman said they were called to the crash on State Highway 1 near Flag Swamp at 11.55am.
Initial indications were that one person had been critically injured. A helicopter was on the way from Dunedin.
The road was closed at the scene and diversions were in place.
The Serious Crash Unit has been advised.