Emergency services are attending a serious crash in the Timaru district, police say.

The two-vehicle crash happened at the intersection of Rangitata-Orari Bridge Highway and Orari-Rangitata Highway (SH79 and SH1) just before 9.30am.

"Two people in one of the vehicles were trapped for a time, and are in critical condition," police said.

A helicopter was called to transport the pair to hospital.

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The road is blocked, and diversions are in place around the scene.

The serious crash unit had been advised, police said.