A witness, Hayden Green, said he had driven to McDonald's at Glenmore Park on Sunday night to buy a sundae when he saw the brutal attack while leaving the drive-through.

Some of the group were armed with cricket bats and golf clubs, said Mr Green, who pulled out his mobile phone and recorded the distressing confrontation about 8pm.

"They were stomping on his head, kicking him in the face, the bats, they were just hitting him everywhere," Mr Green told Channel Nine.

"He looked unconscious. There was a fair bit of blood coming from his face."

Other witnesses who called emergency services said up to 50 youths in the car park were involved in the violence.

Superintendent Brett McFadden, the Penrith Local Area Commander, said that, when police arrived at the fast-food restaurant a short time later, the crowd had already dispersed.

However, three 17-year-old boys returned to the car park and told police they had been assaulted by the group.

One of the boys had a minor head injury, but refused medical treatment.

None of the teenagers would provide police with further information or help with the investigation, Superintendent McFadden said.

Channel Nine identified the person who was filmed being bashed as he lay on the ground as Denzil Parker.

A NSW Police spokesman could not say whether the victim was one of the three teens who spoke to police at the car park shortly after the attack.

Superintendent McFadden said police were relying on information from the public to identify who was involved in the attack.

"We are looking at security camera footage from the fast-food outlet and surrounding businesses ... as well as footage shot on mobile phones," he said.

"This sort of violence is not acceptable and, whether someone is punched or a weapon is used, the potential for someone to die or be seriously injured in a fight is all too real."

He urged a number of witnesses who called triple zero on the night of the attack to contact police again through Crime Stoppers on 1800 333 000.