Jese Smith-Shields was sentenced in the ACT Supreme Court on Friday to a wholly suspended jail term of seven months.

The 22-year-old had been waiting to learn his fate since being found guilty by a jury last month of assault occasioning actual bodily harm.

He argued unsuccessfully during his trial that he had acted in self-defence when he threw the knockout blow at Spilt Milk in 2018, claiming that the injured man had been the real aggressor.

The trial heard that there was tension between Smith-Shields and the victim at the relevant time, as unsubstantiated rumours circulated about the victim sleeping with Smith-Shields' ex-girlfriend.

While the jury ultimately found Smith-Shields guilty of one charge, it cleared him of recklessly inflicting grievous bodily harm in a finding that he had lacked the necessary intent to be convicted of that more serious offence.

Another man charged over the incident, Bayley Loughhead, was found not guilty of both charges.

Justice John Burns said on Friday that while the jury had clearly rejected Smith-Shields' self-defence argument, he was satisfied that the attack had not been premeditated.

Rather, he found, Smith-Shields had made "an instantaneous decision" to punch the victim in "an overreaction" to a heated confrontation.

The judge said there was not enough evidence to support claims that Smith-Shields had come at the victim from the side or behind.

Justice Burns also believed Smith-Shields was remorseful and that he had not intended to cause serious injury, though that did not absolve him of responsibly.

"That is the risk that you take when you punch someone to the head," the judge told the 22-year-old.

The victim told the court last week of suffering from regular night terrors and ongoing pain, and Justice Burns said he would be negatively affected by the incident for a long time.

It was just fortunate, the judge said, that the fallout had not been more serious like in so many other cases that involved blows to such a vulnerable part of the body.

"Luckily, the victim did not sustain a fatal injury or brain damage," Justice Burns said.

Smith-Shields' barrister, Beth Morrisroe, previously urged the judge to deal with the matter by way of a conviction and good behaviour order.

But Crown prosecutor Trent Hickey argued that those things should be accompanied by a suspended jail term to deter others from similarly violent behaviour after such a public incident.

Justice Burns said on Friday that he agreed with Mr Hickey, with any failure to impose a sentence of imprisonment likely to send the wrong message because it would be "unduly lenient".

In suspending Smith-Shields' jail term, Justice Burns said that 22-year-old had strong family support and that he believed the offending was "out of character".

"I consider it unlikely that you will reoffend in the future," Justice Burns told Smith-Shields.

To ensure that he does not have to serve any time behind bars, Smith-Shields must abide by the terms of a good behaviour order for 15 months.