Her vibrant, energetic son - who only hours earlier had been enjoying a night out with mates in The Rocks - had been viciously bashed and she should get to St Vincent's Hospital immediately, the voice at the end of the line said.

She and husband Phillip, of Primbee, were told 26-year-old Simon Cramp had suffered severe head injuries and brain damage and was now in a critical condition.

MORE: "Wrong place, wrong time": man in coma after being caught up in brawl

"When we got to the hospital the news was even worse, that we could be facing the death of our child," Mrs Cramp said.

Since then there have been some positive signs, but Mr Cramp's future is still on hold.

"He's responding to requests to move his hands and feet," Mrs Cramp said.

"He's also struggling to pull out the breathing tubes, so there's obviously brain function."

Police believe Mr Cramp, from Cremorne Point, was simply in the wrong place at the wrong time - standing with two friends under the awning of a McDonald's restaurant in The Rocks around 3am on Sunday morning.

Two groups of men were throwing punches at one another on the corner of Bridge and George streets until security guards and passers-by broke up the fight and they dispersed.

The larger group drifted south, but the smaller one turned north and set upon Mr Cramp, who had been an innocent bystander.

Police have released the CCTV footage seen above and detectives believe the men and women depicted in the footage may be able to assist with their inquiries.

Inspector Greg Jewiss from The Rocks local area command said the victim was repeatedly punched until he fell to the ground, hit his head and lost consciousness.

Mrs Cramp said an ambulance was already in the area, so at least her son got treated quickly.

She is appealing for anyone with information on the attack to contact police.

"There were a lot of people there, a lot of people saw this happen and a lot of people have evidence about who these people are - they need to call the police," she said.

Mrs Cramp called her son's attackers "mindless thugs", and believed there should be zero tolerance for street violence.

"We should be safe on our own streets - not just feel safe but actually be safe," she said.

Mr Cramp grew up largely in Goulburn but attended The Illawarra Grammar School for three years when his family moved to the region.

He now works with a telecommunications company in Sydney.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.