Bradley Mason spent 10 days in hospital with bruising to his brain after he was punched near Subway about 2.30pm on April 19 last year.

Christopher Parlov, 23, is on trial in the ACT Magistrates Court on a charge of committing a negligent act that caused grievous bodily harm.

He has pleaded not guilty.

There is no contest Parlov hit the man, but he argues it was in self-defence as he feared he or his brother would be stabbed.

CCTV footage of the incident was played during the two-day trial, which showed the defendant, his girlfriend and his brother walking through the food court area on level three of the northside mall.

The defendant's brother approached a man whom he had fought with a few weeks earlier and the pair had a heated conversation.

Footage shows Mr Mason and Parlov then move next to the arguing men before the defendant punches the alleged victim in the head with his right fist.

The force of the blow threw Mr Mason off his feet and he hit his head on the floor, knocking him unconscious.

He spent the next week in Canberra Hospital receiving treatment for head injuries

When a Westfield guard, who arrived soon after, asked the defendant what had happened, Parlov allegedly replied: ''These guys jumped us two weeks ago so we're getting them back.''

But Parlov claimed Mr Mason had threatened to stab the pair and he had only punched the alleged victim after he had moved his hands and thought he might have been holding a knife.

The defence lawyer argued his client had first moved to support his brother but then acted in self-defence as he believed his brother faced imminent danger and would be injured if he did not intervene.

The lawyer pointed out that footage appeared to show Parlov throw the punch and then ''try to remove himself from the situation''.

He said the claim his client had cited payback as a motive had been taken out of context.

But prosecutor Michael Reardon said the punch had been ''a simple matter of revenge''.

Magistrate Peter Morrison will deliver his verdict next month.