About 4.15pm police saw a trail bike allegedly travelling in a westerly direction along Northcliffe Dr at high speed.

Police said the rider turned right into Northumberland St, accelerating away from them. Due to the alleged speed of the bike, police did not pursue the rider.

Soon after police received a report that the motorcycle had collided with a Proton Savvy at the intersection of Hertford St and Bedford St, Berkeley.

The 19-year-old motorcyclist was thrown from the bike and hit the car, suffering serious injuries including multiple fractures, a punctured lung and head injuries.

He was airlifted from the scene to Westmead Hospital in Sydney where he is now in a serious condition.

The 48-year-old female driver of the Proton did not suffer any injuries.

Southern Region Crash Investigation Unit attended and a crime scene was established.

Police are appealing for anyone who witnessed the crash, or the motorcycle prior to the crash, to contact Crime Stoppers on 1800 333 000.