One person has suffered serious head injuries and another has moderate injuries after the car they were travelling in hit a tree near Fairlie.

Emergency services were called to the Geraldine-Arundel Highway just after 3am today where a car had left the road and crashed into a tree.

One person with serious head injuries and another with moderate injuries were taken to Timaru Hospital, said a St John southern communications spokesman.

The cause of the crash is not known at this stage.