Police will allege that the critically ill 18-year-old man did not know the 25-year-old who is accused of punching him once in the head in an unprovoked attack, causing the younger man to strike his head on the pavement.

The teenager immediately was knocked unconscious and was not breathing when paramedics were called to the scene about 9.10pm. 

He was taken to St Vincent's Hospital where he underwent emergency surgery on Tuesday night.

Fairfax Media understands that at least one other person was punched and knocked to the ground on Tuesday night on the same section of street just minutes before the 18-year-old was attacked.

The teenager was believed to be coming to the aid of that person when he was hit once in the head from behind. He did not see his attacker coming.

There are believed to be three CCTV cameras on the street, and police are investigating whether the attack was captured on camera.

Police and NSW Ambulance paramedics were on the scene within minutes.

Paramedics took the 18-year-old man to St Vincent's Hospital with critical head injuries. He then underwent surgery and was returned to intensive care, a hospital spokesman said.

The young man's family is with him at the hospital.

Police arrested a 25-year-old man almost immediately following the assault and took him to Kings Cross police station.

He later was charged with maliciously inflicting grievous bodily harm, assault occasioning actual bodily harm and other assault-related offences. He is due to appear at Parramatta Bail Court on Wednesday.

The man was one of 173 people arrested overnight, significantly more than the 97 arrests made last New Year's Eve. Those arrested were charged with 222 offences, including robbery, assaults, street and drug-related offences.

A further 10 people were taken by police to the Sydney City ''sobering-up centre'' - more colloquially known as the "drunk tank".

NSW Police Assistant Commissioner Mark Murdoch said he would not compare last night's king-hit to the fatal attack on Mr Kelly.

"When these things happen they are always a tragedy,'' he said.

"When anyone's assaulted it's a travesty. It is nothing but disappointing for police but also devastating for the young man's family.

"Even with police on every corner, we still cannot stop this kind of fight, particularly when people are affected by alcohol and particularly when they are intent on, we will allege, just going out and hurting people, as we will allege this [person] was."

He said the crowd in Sydney on New Year's Eve was the biggest ever.

"I'm not about to say the crowd was well behaved,'' he said.

"There were a small number of people who didn't heed our warnings, exceeded their limits and came to our attention – and they were swiftly dealt with.

"Intoxication has been the most common issue of the evening and it has been a factor in many of the incidents where charges were laid."

More than 400 people were treated by St John Ambulance volunteers and more than 460 people received assistance from NSW Ambulance paramedics for a range of injuries, mostly due to falls, assaults or intoxication.

On-water revellers had their evening disrupted before they even had a chance to see the 9pm fireworks, when water police had to rescue more than 100 people from a yacht near Garden Island that had started taking on water.

And before the smoke had cleared from that first fireworks display, water police were called out to Farm Cove just in front of Mrs Macquarie's Chair to rescue six people, including two children, from a 30-foot motor cruiser that had overturned and was sinking.

Three people were thrown from the small boat they were travelling in when it collided with another small boat at Middle Harbour near Roseville.

They were taken to Royal North Shore Hospital but were not seriously harmed.

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