A 49-year-old woman has been killed and another woman taken to hospital in a serious condition after two cars collided near Pinjarra, south of Perth.

The accident happened on the South Western Highway, near the intersection of Old Bunbury Road at Coolup, just before 7:00pm (AWST) yesterday.

Police said a 49-year-old woman driving a blue Hyundai Excel crossed onto the other side of the road and hit a maroon Subaru Outback travelling in the other direction.

The 49-year-old from Coolup died at the scene, while a 42-year-old female passenger in the Subaru was taken to Royal Perth Hospital and remains in a stable condition.

Two children, a boy aged 7 and a girl, who were travelling in the Subaru were treated for minor injuries at Peel Health Campus.

It is believe the 43-year-old male driver of the Subaru was uninjured.

Police are asking anyone who witnessed the crash, or observed the Hyundai prior to the crash, to contact Crime Stoppers.