Teenager viciously attacked in Sydney after a vicious robbery in northwest

POLICE are appealing for witnesses to a vicious robbery in Sydney's northwest which has left a teenager in a critical condition in hospital with serious head injuries.

The 15-year-old boy was walking along the Old Bells Line of Road at Kurrajong after leaving a nearby party when he was attacked from behind by at least two people at 12.30am this morning.

He was punched in the head and knocked to the ground before cash was stolen from his wallet.

Another partygoer found him a short time later and helped him to walk home.

From there he was taken by ambulance to Hawkesbury Hospital where he was diagnosed with a fractured skull, eye socket and jaw, as well as bleeding on the brain.

He's since been transferred to Nepean Hospital.

Officers from the Hawkesbury Local Area Command are investigating the assault and would particularly like to speak to anyone who was at the party nearby and has information about the attack.