"My cousin's f---ing dead! He's been stabbed to death," the boy yelled during a triple-0 call, which was played to the jury in an ACT Supreme Court murder trial on Tuesday.

The boy, who had himself been stabbed in the back, told the call-taker he was "bleeding out" as he begged for help.

"Oh my God, I'm gonna die," the boy, aged 16 at the time, said.

When the emergency line operator asked whether the 16-year-old could perform CPR on his 18-year-old cousin until paramedics arrived, the boy replied: "No, he's not talking. He's dead. He's completely dead!"

Before the boy eventually stopped responding, he repeatedly said he needed an ambulance and, at one stage, cried out that "it's all my fault".

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The court has previously heard that, during a two-minute melee involving 12 people at the Weston Creek skatepark in September 2020, the 16-year-old was stabbed in the back by a boy then aged 15.

That boy, now aged 17, has pleaded guilty to a charge of recklessly inflicting grievous bodily harm on the 16-year-old.

But he has pleaded not guilty to a murder charge and denied responsibility for the death of the 18-year-old cousin, who was stabbed six times during the fight.

Neither the alleged killer nor the cousins can be named for legal reasons.

After the triple-0 call had been played on Tuesday, the fourth day of the trial, prosecutors showed the jury an interview the surviving cousin did with police about 36 hours after the melee.

In the recording, the boy told police multiple times about how a Snapchat argument, which culminated in the skatepark fight, had left him "so angry". Details of the social media stoush have been suppressed.

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"I had so much anger and adrenaline," the boy said from a bed in Canberra Hospital, where he spent about three days recovering from his injuries.

The boy told police he had believed one-on-one fights would occur at the skatepark in response to the Snapchat war of words, and that there would not be any weapons involved.

He said that once he arrived, however, he was pulled from the front passenger seat of a car and "absolutely pummelled" by multiple people.

The boy described seeing members of the opposing side brandishing weapons during the fight, which felt like it had lasted "forever".

He also spoke of cradling his cousin after being told in the aftermath the 18-year-old was dead.

He described how he then panicked and ran to the nearby Weston Creek McDonald's in search of someone who might know how to do CPR.

The boy said "there was no one", so he began running back towards his cousin.

However, he was bleeding heavily from his own injuries and he collapsed in some grass on his way back to the skatepark.

"I couldn't get up and my vision went blurry," the 16-year-old said.

Later on Tuesday, the jury began watching a second police interview with the boy.

The trial continues.

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