Reece Watherston is accused of killing 22-year-old Jack Hanley during an early morning brawl on Hindley Street in 2017.

Giving evidence on Monday, Watherston told the Supreme Court he was thrown onto the road before his confrontation with Mr Hanley.

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

"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'."

Fighting back tears from the witness stand, the 21-year-old said he was dodging Mr Hanley and started swinging back at him.

He said he aimed his punch at Mr Hanley's face "to get him off me and away from me... I had to protect myself".

"It was quite hard, he was a big guy," he said of the blow.

"I was scared, we were all scared."

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

"I just wanted to get the hell out of there, in case his friends may have come at me," he said.

Earlier in the trial, prosecutor Mark Norman SC said Mr Hanley died from catastrophic head injuries caused when he fell backwards and fractured his skull on the concrete pavement.

The court heard Watherston had engaged in a drunken brawl that initially involved two groups of people, but Mr Hanley posed no threat at the time of the blow.

It was also told that both Mr Hanley and Watherston had the drugs MDMA and cannabis in their systems.

A post-mortem examination found Mr Hanley died from blunt head trauma which caused a severe brain injury.

The trial is continuing before Justice Anne Bampton in the absence of a jury.

Australian Associated Press