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.

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The Serious Crash Unit has been advised.