Major crash investigators are piecing together what happened in a car park that left a 30-year-old pedestrian with serious injuries.

Police said about 3pm Thursday, a Mazda hatchback collided with a pedestrian in the car park of a beach in Mosman Park, west of the city.

She was seriously injured and taken to Royal Perth Hospital.

The driver of the hatchback, a 30-year-old man, has been assisting police with inquiries.

Anyone with information on should contact Crime Stoppers.

Footage from mobile or dash cam can be uploaded to investigators here.