The crash happened when two vehicles travelling north on Appin Road collided about 10.50pm.

Initial reports indicate a Ford Mondeo had slowed to turn right when a white Mitsubishi Magna sedan struck it from behind.

A two-and-a-half-year-old boy, travelling in the Magna, sustained serious head injuries and was airlifted to The Children’s Hospital, Westmead.

Police have been told he is in a critical condition with multiple fractures.

A 27-year-old man driving the Magna was taken to Liverpool Hospital with head and chest injuries.

Police do not believe the driver, from St Helens Park, was wearing a seat belt at the time of the crash.

A man and woman, aged 57 and 49 both of Appin travelling in the Ford, were taken to Campbelltown Hospital to be examined as a precaution.

Appin Road was closed between St Helens Park and Appin between 10.30pm and 3am while Crash Investigation Unit police examined the scene.

Anyone who witnessed the crash or who has information about the incident, is asked to contact the Crash Investigation Unit or Crime Stoppers on 1800 333 000.