Police are appealing for information about a crash in North Boyanup where two cars collided head on before one of the vehicles slammed into a tree.
The crash closed the South Western Highway for two hours on Sunday after a Mazda utility travelling in a northerly direction crossed onto the wrong side of the road and collided with a Nissan utility travelling in the opposite direction.
Police said the vehicles collided head on at about 11:30am with the impact causing the Mazda to be forced off the road and it colliding side on with a large tree.
A 68-year-old woman driving the Mazda was cut from the vehicle by Department of Fire and Emergency Service personnel and flown to Royal Perth Hospital by a rescue helicopter with serious injuries.
A female passenger in the Nissan was taken to Bunbury Hospital with non-life threatening injuries.
Police would like to speak to anyone who witnessed the crash or saw the vehicles driving prior to the crash.
Contact Crime Stoppers on 1800 333 000 or make report online at www.crimestopperswa.com.au