Two seven-year-old girls have been killed after the car they were in struck a tree in rural Western Australia.

The children died on Sunday morning at the crash scene at Carrabin, about halfway between Perth and Kalgoorlie.

The driver, a 31-year-old woman, was taken by St John paramedics to Merredin Hospital.

Police, who were alerted to the incident about 11am by paramedics, have appealed for dashcam footage to help them understand how the crash occurred.

Dashcam or mobile phone footage of the incident can be uploaded to a dedicated page on the WA police website.

The crash closed the Great Eastern Highway for many hours, cutting off a vital route through the Wheatbelt region.

Meanwhile, an Adelaide man is in a critical condition in hospital after he was struck by an SUV on the Port River Expressway on Sunday afternoon.

The driver, a 71-year-old man from Enfield, was not physically injured but was taken to hospital for mandatory testing.

-AAP