UPDATE: The driver, Mr Tenny, later told police that he had swerved to avoid a cat running across the road.
Police were called to the scene at around 2:45am.
Officers from Wichit Police Station and rescue workers arrived to find that the driver, named as British national Jack Richard Tenny, had sustained minor injuries.
His passenger, Danielle Joyce Swaby, also from the UK, had suffered more serious injuries and was transported to Dibuk Hospital by rescue workers.
Mr Tenny told police he was driving from Kathu towards Chalong at the time of the accident.
Officers documented the scene with photographs and a site sketch. An investigation into the cause of the crash is ongoing, police said.