"How do you react when it looks like someone's being killed right there in front of you?"

A murder-accused inmate's lawyer posed that question to jurors as horrific footage of a maximum-security jail yard killing was replayed in court.

Paul Simon Tuliloa, Riki Wiremu Ngamoki and Lopeti Telefoni have all pleaded not guilty to the murder of inmate Blake John Lee at Auckland Prison in Paremoremo.

A fourth inmate, who has name suppression, stomped on Lee and stabbed him with a shank in a prison yard on March 5 last year.

And that shank-wielding inmate was on a probably unstoppable quest to kill, Ngamoki's defence counsel Anoushka Bloem told the High Court in Auckland today.

Bloem said her client was not in a position to stop the stabbing, but he did not help the murderous inmate either.

She told the court Ngamoki had no knowledge about any planned attack on Lee.

And Bloem said her client's involvement in the violence was restricted to a simultaneous but separate attack on Lee's fellow Mongrel Mob inmate, Cesar Su'a.

She said even if Ngamoki's assault on Su'a coincidentally helped the shank-wielding inmate, that was insufficient to make Ngamoki guilty of murder.

"This was no group attack. There was no plan. It was spontaneous."

Bloem said the young men in the yard had virtually nothing to do expect maybe exercise and play cards.

Jurors have been told Ngamoki and Tuliloa identified with the Killer Beez and Telefoni was with the Crips.

The Paremoremo inmates were locked up 23 hours a day and there was "not even a ball for the basketball hoop" in the yard, Bloem said.

Prosecutors have argued Ngamoki fought with Su'a to prevent him helping Lee.

But Bloem said the fourth inmate was on a ferocious, individual mission.

"He showed absolutely no restraint in his frenzied killing of Mr Lee."

She said this meant Su'a could probably not have helped Lee anyway.

And jurors heard the inmate with the shank stabbed Lee another 15 times after Ngamoki had disengaged from the fight with Su'a.

Bloem said if prison security was functioning properly, the fourth inmate would never have been allowed to smuggle a shank into the yard.

She said nobody, including Ngamoki, had reason to believe a lethal weapon such as the shank would have been in the yard.

But she said the Crown had effectively told Ngamoki: "You must have seen it and therefore you must be guilty."

Prosecutor David Johnstone previously suggested the presence of Mongrel Mob members was agitating Ngamoki for some time before the violence broke out.

The three men on trial have admitted injuring with intent.

Telefoni has admitted throwing the first punch at Lee.

Yesterday, his lawyer Emma Priest said Lee flashed a Mongrel Mob hand gesture in the prison yard, and that gesture provoked a violent reaction.

Tuliloa's lawyer Gary Gotlieb is expected to give his closing address later, before Justice Simon Moore sums up the case to jurors.

The trial continues.