Two people have died in a crash in Woodend, north of Christchurch.

A four-wheel drive was towing another vehicle on a trailer when it was involved in a head-on collision with another car, said police Inspector Peter Cooper.

Police and emergency services were called to the scene on State Highway 1 around 3.45pm.

Two people travelling in one of the vehicles died, police said.

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One person who was trapped in a vehicle is in a serious condition and was flown to Christchurch Hospital.

Another passenger has moderate injuries and was also taken to Christchurch Hospital,
a St John Ambulance spokeswoman said .

The Serious Crash Unit will be investigating.

Diversions were in place and motorists were encouraged to take alternative routes.

The New Zealand Transport Agency said SH1 was closed near the intersection with Sandhill Rd.

Southern fire communications shift manager Tim Reynolds said four fire trucks attended.

"We've got appliances from Woodend, Kaiapoi and Rangiora and we have assisted with rescue equipment with one person," he said.

A witness said one ambulance and one helicopter had just left the scene.