The mother of a one-punch attack victim who was fatally struck during a night out in Adelaide's Hindley Street has broken down outside court, after the young man who inflicted the blow was found not guilty of manslaughter.

Key points: Jack Hanley was fatally hit in Adelaide's CBD in December 2017

Jack Hanley was fatally hit in Adelaide's CBD in December 2017 Reece Watherston was charged with manslaughter but claimed his actions were self-defence

Reece Watherston was charged with manslaughter but claimed his actions were self-defence A judge concluded Watherston delivered the fatal blow but could not find him guilty

Jack Hanley, 22, died in December 2017 from catastrophic head injuries inflicted when he was hit during a drunken brawl on the corner of Hindley and Morphett streets, on the city's nightclub strip.

Reece Andrew Watherston, who was 20 years old at the time, was charged with manslaughter, but claimed he was acting in self-defence.

Mr Watherston told South Australia's Supreme Court he never wanted to fight, but felt scared for his own safety after being knocked to the ground.

The prosecution alleged Mr Hanley was backing away when he was hit, but was struck with such force he would likely have been unconscious before his head hit the concrete pavement.

In delivering her verdict, Justice Anne Bampton accepted that Mr Watherston inflicted the punch that caused Mr Hanley's death.

"The punch by Mr Watherston constituted an unlawful physical assault upon the deceased carried out in circumstances where a reasonable person would have realised that he was exposing the deceased to an appreciable risk of serious injury," she said, in her written reasons for verdict.

However, she said the prosecution had not satisfied her beyond reasonable doubt that she should reject Mr Watherston's version of events.

Reece Andrew Watherston refused to speak as he left court following the verdict. ( ABC News )

"The prosecution has not excluded as a reasonable possibility that Mr Watherston, when confronted by Mr Hanley at approximately 5:20 am on 10 December 2017, fought back because he believed he had to, and that that belief was reasonable and genuine, and that his conduct was proportional to the threat he perceived," Justice Bampton said.

"The prosecution has not established beyond reasonable doubt that Mr Watherston was not acting in self-defence when he punched Mr Hanley causing his death."

'Isn't my son's life worth more?'

It was standing room only in the packed courtroom, and many of Mr Hanley's family members reacted angrily, with his mother crying out "no".

The fatal punch was caught on security vision, but Justice Bampton declined to release it to the media.

Mr Hanley's mother Julie Kelbin was distraught as she spoke outside court, and said her family had been denied justice.

"While I visit a cemetery to sit at my son Jack's grave, Watherston gets to live his life freely," she said, while fighting back tears.

"Where is the justice for my son Jack, who had never had a violent fight in his life?

Mr Hanley's mother Julie Kelbin was devastated and angry at the outcome. ( ABC News )

"How many more deaths on our street is it going to take before our legal system starts to take this seriously?

"This judge has just said 'go and live your happy life while this mother lives in misery'.

"How can a person kill another person with one punch [and] just receive a f***ing not guilty? Isn't my son's life worth more than this?"

Ms Kelbin described her son as a "gentle soul" who had "the rest of his life" ahead of him.

"Myself and my family have had to live without this missing link. I'll never get to see my son marry and have his own children. I have been robbed," she said.

"I've lived 568 days of hell without my youngest son Jack. I know the offender didn't intend to kill my son but he did. Where is the justice?

"Every weekend someone is hurt in this city, when are we going to step up?"

Friend says Jack Hanley 'wasn't fighting'

A friend of Mr Hanley, Kalise Pansini, said a fight had broken out on the night Mr Hanley was killed, but he was not involved.

"Jack wasn't fighting, he was standing next to me," she said.

"The sound of his head hitting the pavement doesn't leave my mind.

"I can't believe it, I can't believe we've gone through what we've gone through and they can stand there and say he's not guilty."

She described her friend as "the best person" and said he "[didn't] have a nasty bone in his body".

When Mr Watherston left court as a free man, members of the victim's family cried out at him.

Police officers gather evidence at the scene in December 2017. ( ABC News: Simon Royal )

On his way into court before the judgment Mr Watherston told the media his thoughts were with Mr Hanley's family.

"Regardless of the verdict today my thoughts and condolences will always be with the Hanley family," he said.

"The 10th of December 2017 is something that will remain with mine and my family's life forever."

Mr Watherston remained silent as he walked out of court.