Police are seeking witnesses after reports a car veered across the centreline outside Christchurch this morning and crashed head on into a truck and trailer.

The driver of the blue south-bound car, plus the driver of a van who was following the truck, were trapped in their vehicles for up to 30 minutes after the crash on State Highway One, near Templeton, at around 11.20am.

The car's driver was taken to Christchurch Hospital in a serious condition, while the van driver was also hospitalised in a moderate condition.

The driver and passenger of the truck and trailer unit were uninjured but badly shaken, police said.

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State Highway One Main South Road was closed and traffic diverted for more than two hours while police and emergency services cleared the scene.

Police investigations into the crash are underway.

Anyone who witnessed the crash is asked to contact Rolleston Police Station on 03 347 9248 and leaving a message with their contact details.

Information can also be provided anonymously by calling Crimestoppers on 0800 555 111.