Liam Sharman, an 18-year-old p-plater from Kearsley, had been charged with a number of serious offences, including dangerous driving occasioning grievous bodily harm and failing to stop and assist after vehicle impact occasioning grievous bodily harm, after a 19-year-old man, who police said had been a friend of his, fell from the roof of a BMW he was driving outside an 18th birthday party at Mayfield on December 22 last year.

Sharman, who is represented by solicitor John Anthony, appeared in court on Wednesday and pleaded guilty to failing to stop and assist after impact causing injury and grievous bodily harm by negligent act after the DPP agreed to withdraw the more serious charges.

According to court documents, Sharman and a friend were speaking to people outside the party when two men, including the victim, asked to be driven to a nearby bottle shop.

It was on the return trip that, for no apparent reason, the victim began punching Sharman and his friend in the back of the head.

When they got back to the party the victim had to be forcibly removed from the car. Sharman left but returned a short time later when he realised the victim had left his phone behind.

The BMW was travelling slowly on Scholey Street when the victim ran towards it and, in an "aggressive manner", jumped onto the bonnet and started stomping on the windscreen.

Sharman kept driving slowly as the victim lay across the roof and used his fists to bash the windscreen.

Sharman later said he was driving slowly but admitted to crash investigators that he swerved left and right and then turned into Valencia Street. The victim then fell from the roof of the car and struck his head on the road, causing a traumatic brain injury.

He was taken to hospital in a critical condition and has no memory of the night.

Sharman kept driving and didn't stop or call emergency services.

Witnesses later described Sharman's car driving quickly and one said it became airborne over a speed bump, causing the victim to fall off.

"We were going to stop, but had not fully stopped yet and he ran on top of the bonnet, caving the windshield in," Sharman said. "He got on top of the car, while we were still slowly moving. And I put my foot on the brake. I didn't slam it on. We were still moving... and he came off. I didn't know he was hurt, I was scared what would happen if he got up and kept going."

Sharman will be sentenced in December.