A man is dead and a woman seriously injured after a car crash in the Hurunui District, north of Christchurch.

A car rolled off State Highway 7 around 5am on Karaka Rd, about 1km south of Mt Alexander.

The vehicle appears to have left the road at speed, gone through a fence and rolled into a paddock, police said.

The injured woman was flown to Christchurch Hospital.

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Police said they wanted to thank those who stopped at the scene to offer assistance.

The Police Serious Crash Unit is investigating.