A man has been charged over a one-punch hit in Canberra's city centre on New Year's Day after video of the attack was released to the public.

The 20-year-old Braddon man handed himself in to the City Police station this afternoon, and had been assisting police with their investigation.

He has been charged with one count of recklessly inflicting grievous bodily harm.

The man was released on bail and will appear before the ACT Magistrates Court on January 15.

The victim of the attack, aged in his 20s, had emergency surgery following the punch, which happened outside a convenience store on East Row in Civic about 2:45am on New Year's Day.

In the CCTV footage, the victim is seen to fall sideways, hitting his head on the concrete, before another man rushes forward to support his head.

Constable Jarrad Drennan said the victim was lucky to be alive and the outcome could have been far worse.

"He's obviously in a fair bit of pain, but he's extremely lucky," he said.

"We're lucky that we're not standing here today talking about a death.

"He's lucky, if not a miracle that he didn't land a different way.

"The footage you've all seen is sickening."

'I heard the crack'

Witness Bec Hunter said she was shocked by the attack.

"I heard the crack, and that's what got me to turn around, and I saw him as he fell," she said.

"You don't forget sounds like that ... it sticks in your head.

"He was completely unconscious, unresponsive, his eyes weren't moving, hands weren't moving, and cold."

Another witness Dan McDougall said the attack was unexpected.

"I didn't hear any commotion or fighting or any sort of back and forth between anyone," he said.

"It took us a few seconds to sort of click in that there was this person on the ground."

Officers said when they arrived at the scene, they found the victim laying on his back unconscious.

Police said he sustained injuries that required him to undergo emergency surgery, including surgery to his jaw.

The victim has been discharged from hospital and is now recovering at home.