The 17-year-old accused pleaded guilty in the ACT Supreme Court on Thursday to a charge of recklessly inflicting grievous bodily harm on a boy aged 16 at the time in question.

He then went on trial, having pleaded not guilty to a charge of murdering the 18-year-old cousin amid the two-minute melee.

Neither the accused nor the cousins can be named for legal reasons.

In her opening address to the trial jury, Crown prosecutor Rebecca Christensen SC said the September 2020 fight occurred on the back of a "heated" Snapchat argument between two groups of teenagers.

She said their "quite aggressive" communications led to an agreement to meet at the Weston skatepark, in the early hours of a Sunday morning, for what was expected to be a fist fight.

However, adult men were called upon to attend as "back-up" for one of the groups and things "escalated", as Ms Christensen put it, once 12 people had turned up at the skatepark.

She said the 16-year-old victim was quickly dragged from the car he had arrived in and attacked by people including the accused, who has now admitted stabbing him in the back.

Ms Christensen said that around the same time, on the other side of the car, the boy's 18-year-old cousin was set upon by three men, who only stopped assaulting him when they saw another of the fight's participants waving a machete around.

The adults retreated to their vehicle to fetch "gardening implements" with which they damaged the cousins' car, and Ms Christensen said it was most likely at this point of the "quick and confused melee" that the accused fatally stabbed the 18-year-old.

She told the jury the deceased, who sustained six stab wounds, had died at the scene despite paramedics having raced there and attempted to revive him.

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The prosecutor alleged that forensic evidence linked the accused, who was 15 at the time of the incident, to the stabbings of both cousins.

Ms Christensen also said the accused had made multiple admissions in the aftermath, which had included speaking about stabbing "them" and telling someone he had "stabbed about seven times".

She indicated she would rely on a circumstantial case in her bid to prove the murder charge, saying she did not believe any participants in the melee would give evidence of having seen the accused stabbing either cousin.

However, she said the accused was the only person who had attended the skatepark with a knife that could have caused the cousins' injuries, and that experts would testify it was possible the same weapon had been used to stab both victims.

In brief opening remarks on Thursday afternoon, defence barrister David Barrow urged jurors not to "rush to judgement" and stressed that his client was "just a boy".

He went on predict that jurors would find "a great number" of the Crown's witnesses were people "quite willing to lie".

Mr Barrow said along with the "reliability and truthfulness" of the witnesses, key issues in the trial would include the opportunities others had to inflict the deceased's fatal injuries.

He told jurors "an arsenal of knives" had been found at the home of one of the adult fighters.

Mr Barrow is set to continue addressing the jury of 15 on Friday morning.

Chief Justice Lucy McCallum has told jurors the trial is expected to take five or six weeks.

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