Police are asking anyone who saw a 13-year-old girl step off a moving bus in Cockburn to come forward.
The girl was on a privately owned bus travelling south on North Lake Road just after 3.30pm on Monday when she stepped off the back while it was still moving at low speed.
The teenager fell to the ground and knocked her head.
A woman travelling behind the bus stopped and helped the girl but she refused medical attention and asked to be taken to Cockburn Train Station.
Police say her health has since deteriorated and she's currently in hospital receiving treatment.
Crash investigators wish to speak to anyone who witnessed the incident, particularly the woman who assisted the victim and drove her to the train station.
Anyone with information or dash-cam footage of the incident is asked to call Crime Stoppers on 1800 000 333 or report online www.crimestopperswa.com.au.
Early voting for Secret Harbour by-election begins Monday
Welfare concerns for missing Swedish national last seen in Warnbro
Wanted man last seen in Golden Bay
Passenger critically injured in West Pinjarra crash
Unregistered painter fined for requesting hefty deposit from Singleton homeowner and not doing the job
Police hunt driver following dangerous Baldivis pursuit
Welfare concerns for boy last seen at Port Kennedy skate park
Rubbish truck catches fire in Baldivis
Driver injured in bus crash in Rockingham