A 24-year-old man has been sentenced to more than three years' jail over a one-punch attack that left the victim on life support.

Earlier this year Daniel Angelos was found guilty of recklessly causing grievous bodily harm over the assault in Sydney's CBD.

Angelos attacked Simon Cramp at the Rocks in June 2013, causing him to fall and hit his head.

Mr Cramp, 26, sustained critical injuries and was put on life support.

Mr Cramp had a section of his skull removed to limit brain damage after Angelos punched him three times, including a major punch to the jaw.

In sentencing, Justice Andrew Haesler acknowledged that Angelos had turned himself in and tried to plead guilty.

But he added the sentence must also be a deterrent and sentenced him to three years and four months in prison.

With time already served, Angelos could be released on parole next August.

In a victim impact statement read to the hearing, Mr Cramp told the court his life had changed irrevocably since the night he was hit in the jaw.

Simon Cramp and his family after the sentencing. ( ABC News: Jessica Kidd )

He said he has suffered ongoing physical and mental health problems as a result, but he had tried not to let it dictate his life.

Mr Cramp told the sentencing hearing he still had health problems but his recovery had been beyond expectation.

He told the court he felt strongly connected to the families of other so-called one-punch victims who had not been so lucky, and Angelos should not have the same fortune in sentencing he has had during his recovery.

Mr Cramp's mother Angela said she was lucky her son was still alive, but other families - such as that of Thomas Kelly who died in 2012 after he was punched by Kieran Loveridge in Sydney - have not been so fortunate.

"We're standing here with Simon and his attacker got three years and four months," she said.

"Thomas Kelly's parents stood outside the court without their son. Kieran Loveridge got four years.

"So you have to put it into perspective. Obviously people are starting to see a need to do something."