Two people are in a critical condition and a third has serious injuries after a crash in Southland this morning.

Two cars collided at the intersection of Rimu Rd and Longbush South Rd in Southland around 10am.

Police said one person was trapped in one of the cars and was in a critical condition.

Another patient with a head injury was also in a critical condition.

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A third person had serious injuries.

A St John Ambulance spokesman said all three patients were being transported to hospital.

"But it's not clear which hospital yet," they said.

Police cordons were in place at the scene and motorists were being asked to be patient.