Two inmates who brutally attacked a fellow prisoner, exacting more than 70 blows in less than a minute, have failed in a bid to have their sentences reduced.

Ethan Hokai, 24, and Kodie Reece Russell, 22, each had 18 months added to their existing jail terms when they were sentenced on a charge of assault with intent to injure at the Dunedin District Court in September last year.

They each took their dispute over that outcome to the High Court earlier this month and Justice Gerald Nation recently released his judgment declining the appeal.

At the time of the attack, in April 2019, Hokai was serving a term of five years three months for aggravated robbery and injuring with intent to commit grievous bodily harm, while Russell was doing three years for dishonesty, arson and drug offences.

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Russell's counsel Anne Stevens QC argued the violence used during the incident was only moderate and "seemed to be directed more to showmanship than injury".

The attack was relatively short, she said, and the victim avoided serious injury.

Justice Nation accepted it had been "an orchestrated attack", which started when Hokai approached the victim and kicked him.

Simultaneously, Russell punched the inmate in the side of the head.

"It was a sustained attack," the judge said.

CCTV footage of the brawl showed Hokai punched the victim 33 times to his head and body, then kicked and kneed him eight times as he tried to protect himself.

Russell, the court heard, was responsible for 20 punches, as well as 12 knees and kicks.

Judge Dominic Flatley's description of what happened as "a fairly brutal attack" was conservative, Justice Nation said.

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Court documents stated the victim suffered a swollen lip, abrasions to his face and a loose tooth. The lack of severe injury was "most surprising", the judge said.

Hokai explained the rationale behind the violence.

He said the victim had "got smart" to him but he acknowledged the beating was a "stupid idea".

Both defence counsel argued the extra prison sentence imposed on their clients, combined with two weeks of sanctions faced behind bars was double punishment.

Crown prosecutor Richard Smith said the loss of privileges and safe management of prisoners was a matter for Corrections. There was no legal authority for that to be taken into account by the court, he said. Justice Nation agreed.

"Mr Hokai and Mr Russell would have known when they were in prison that offending of this sort would bring inevitable penalties within the prison system of the sort they were subjected to," he said.

"That obviously did not deter them from assaulting the victim as they did."