An Auckland teen has admitted stabbing a promising rugby league player in the neck with a broken bottle but says he did not intend to kill him.

LISTEN ABOVE: NZME reporter Rob Kidd speaks to Larry Williams about the events of the trial today. 

Vincent Angene Skeen, 17, is on trial in the High Court at Auckland charged with murdering 17-year-old Luke Tipene.

Neither of the boys had attended the Grey Lynn party where the incident happened but were there to pick up friends in the early hours of November 1 last year.

Crown prosecutor Zannah Johnston said the gathering at a house on Great North Rd spilled out on to the street where a disagreement between two men -- brewing throughout the night -- resulted in a "one-on-one" fight.

The pair began to trade blows when Skeen allegedly intervened, punching Mr Tipene's friend in the back of the head.

Ms Johnston said Mr Tipene responded by punching Skeen, who ended up on the ground.

While there, the Crown says the defendant grabbed a broken bottle.

"Vincent Skeen stood back up, clearly enraged, and was not going to leave matters at that," Ms Johnston said.

She told the jury the teen rained down blows on the "dedicated and promising" athlete in a downward "hammering" motion.

It is alleged Skeen hit Mr Tipene with the sharp bottle neck at least seven times.

Ms Johnston said a pathologist would give evidence of 13 sharp-force injuries to the victim's body.

The fatal blow was nearly 12cm deep and pierced Mr Tipene's jugular vein on the left side of his neck.

Bleeding profusely, an ambulance rushed him to Auckland City Hospital, but he died at 4.45am.

Skeen's lawyer Lorraine Smith accepted her client was responsible for killing the teenager but said he was not guilty of murder.

"At the time Vincent swung out holding a broken bottle, he had no appreciation his actions could or would cause another boy to die," she said.

Mrs Smith said Mr Tipene was "bigger and stronger" than Skeen and had punched him to the ground twice before the fatal blow.

She conceded the defendant's actions were "very foolish" but told the jury they would have to consider whether he intended to connect with the victim.

"If Vincent is guilty of anything, he's guilty of manslaughter," Mrs Smith said.

One of the first police officers on the scene described the aftermath of the melee.

Constable Ray Fa'aofo said he had to calm down a couple of angry young men before turning his attention to Mr Tipene, who was lying on the pavement while someone applied pressure to his neck with a jacket.

Mr Fa'aofo said he immediately realised the teen was in trouble.

"There were pools of blood underneath him. I knew it was quite serious because he had lost a lot of blood," he said.

When he took over applying pressure to the wound, he said Mr Tipene started coughing up blood as his eyes rolled back.

The trial, before Justice Mark Woolford and a jury of six men and six women, is expected to last up to three weeks.