Woman dead after fatal crash in Perth's south

A woman has been killed and three others seriously injured in a horror head on collision in Perth's south.

Some of the wounded were trapped for hours as rescue crews worked to free them on Armadale Road in Piara Waters late yesterday.

A 34-year-old woman, the driver of the Suzuki wagon, suffered head and arm injuries and was taken to Fiona Stanley Hospital.

Her 34-year-old male passenger was flown to Royal Perth Hospital with pelvic and leg injuries.

The 52-year-old man driving the bus suffered just minor leg injuries.

The woman driving with him, aged 51, was rushed to Royal Perth with serious pelvic, abdominal and leg injuries but couldn't be saved.

All the survivors have now been confirmed as stable.

Major Crash officers are investigating why the two cars collided around 5.20pm yesterday.

They are now calling for anyone who may have seen the grey Suzuki Vitara or orange Toyota Coaster Bus in the Piara Waters area, prior to, or at the time of the crash.

Anyone with information should call Crime Stoppers on 1800 333 000.