That is what Chief Justice Lucy McCallum said in the ACT Supreme Court on Tuesday, when she also labelled organised fights between youths "a problem in this city".

The territory's top judge spared the 17-year-old offender further time behind bars, sentencing him to a partially suspended 13-month jail term.

The boy, who was just 15 when the deadly fight took place in September 2020, had previously pleaded guilty to a charge of recklessly inflicting grievous bodily harm.

In sentencing, Chief Justice McCallum described how the boy, who was affected by alcohol and drugs, had attended a birthday party on the night in question.

One of the teenager's friends became involved in a Snapchat argument with some strangers during the party, and the offender was invited to come along to a physical fight that had been arranged in response to the social media stoush.

He accordingly travelled to the scene at the Weston Creek skatepark in a car full of minors, one of whom had contacted a group of men to attend as back-up.

Three vehicles, carrying 12 people, none of whom can be named for legal reasons, ultimately converged on the agreed meeting place.

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There, Chief Justice McCallum said, "a violent fight began immediately and escalated quickly".

During the brief melee, which lasted about two minutes and 12 seconds, an 18-year-old man and his 16-year-old cousin were both stabbed.

"One survived. One did not," Chief Justice McCallum said.

The 17-year-old offender was accused of stabbing both, but, in June, a jury found him not guilty of murdering the 18-year-old man after a trial that spanned more than five weeks.

The boy gave evidence in his own defence during his trial, saying he had picked up a knife from the ground mid-brawl and used it to stab the 16-year-old in the back because he thought the victim was "bashing" his friend.

This "unplanned and impulsive" act inflicted what Chief Justice McCallum called "a long and deep gash", which has left a scar on the 16-year-old's back.

The judge said it was likely the 18-year-old murder victim had already been stabbed by an unknown assailant with the same weapon, which the offender later stashed in his pants.

After learning several hours later that someone had died during the brawl, the offender, at that stage under the impression he had killed someone, threw the knife down a drain.

The boy told his trial he later found out who the deceased was, and realised it was someone he had not been anywhere near during the fatal fight.

Between his arrest in October 2020 and June 9 this year, when he was granted bail a second time, the offender spent a little more than seven months behind bars on remand.

Chief Justice McCallum said he had also been subject to strict conditions during the periods he had spent on bail, taking into account "quasi-custody" as she found his sentence should be backdated by 10 months.

The judge said it was "common ground" between prosecutors and the offender's lawyers that the boy need not go back into custody.

Defence barrister David Barrow asked last month for a jail sentence amounting to time served, with no further period of imprisonment and simply a good behaviour order going forward.

But Crown prosecutor Rebecca Christensen SC sought further imprisonment, albeit suspended, and a good behaviour order involving a lengthy period of supervision.

Chief Justice McCallum opted on Tuesday for the latter, describing it as a "carrot and stick approach" that would leave the prospect of further time behind bars hanging over the teenager's head if he got into trouble again.

She accordingly imposed the 13-month jail term, suspending the remaining three months immediately and attaching a two-year good behaviour order.

"Provided you comply with the conditions of that order, that will be the end of it," the judge told the boy, who was supported in court by his father.

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