An Adelaide man on trial for manslaughter has been found not guilty over the one-punch death of a 22 year old during a brawl in 2017.

Reece Watherston was accused of killing Jack Hanley during an early morning fight on Hindley Street.

Watch above: Decision handed down in one-punch manslaughter trial

The court erupted as Justice Anne Bampton handed down her verdict, finding the DPP did not disprove Watherston's claim of self defence beyond reasonable doubt.

Hanley's mother yelled out "no" as a swell of supporters tried to exit the courtroom.

Outside the court, she delivered a highly emotional statement.

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

"I will never see my son marry and have children."

'Drunken brawl'

Earlier, Watherston told the South Australian Supreme Court he was thrown onto the road before his confrontation with Hanley.

"Out of nowhere I've seen this huge, huge male approach me with clenched fists swinging at me," he said.

RELATED: One-punch manslaughter accused tells Supreme Court he was 'scared'

"I just remember seeing his fist go past my face and that's when I thought 'holy s*** I need to pay attention to what's going on here'."

Prosecutor Mark Norman SC said Hanley died from catastrophic head injuries caused when he fell backwards and fractured his skull on the concrete pavement.

Watherston said he did not remember seeing Hanley fall and immediately left the scene.

For more SA news, click here.