Waikato Police are appealing for witnesses after a blue Mercedes crashed into a tree at Horotiu on State Highway 1 at 10.45am.

The driver of the car has critical injuries and the front seat passenger has moderate injuries.

The road is closed but is expected to reopen shortly.

Diversions are in place.

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Sergeant Charles Burgess said anyone who saw the crash or the car was in the area at the time was urged to call Hamilton Police on (07) 858 6200.

St John Ambulance says two patients were taken to Waikato Hospital.