Two adults and two children have suffered serious injuries in a crash south-east of Mandurah.
Around 9.37am on Sunday, a beige Mitsubishi Pajero was driving north-east on Pinjarra-Williams Road in Williams when it left the road and hit a tree.
The 44-year-old male driver and a 42-year-old female passenger were airlifted to Royal Perth Hospital with serious injuries.
A 10-year-old girl and an eight-year-old boy also suffered serious injuries and were taken by St John Ambulance to Narrogin Hospital.
It is understood they will both be airlifted to Perth Children's Hospital today.
Major Crash Investigators are conducting an examination of the scene and are appealing for any witnesses to contact Crime Stoppers on 1800 333 000 or make a report online via crimestopperswa.com.au.
Dash-cam or mobile phone vision relating to the crash can be uploaded to investigators directly via the below link:https://wapf.au.evidence.com/axon/citizen/public/serious_crash_williams