The incident happened on Sommerville Road in Hornsby Heights just after 10.40am today.

Police said the young girl was found unconscious on the road after she was hit by a Holden SUV.

CareFlight’s specialist doctor and NSW Ambulance paramedics treated the girl for serious head injuries before she was taken to hospital in a critical condition.

A statement said she had been hit by a car traveling at speed while she was crossing the road.

The driver of the SUV was uninjured and taken to Hornsby Hospital for mandatory testing.

A crime scene had been established and police have started an investigation.