Romeo Te Hei Williams, 24, appeared in the Dunedin District Court yesterday after earlier pleading guilty to injuring by unlawful act.
The court heard the defendant was serving a lengthy sentence for being a party to an aggravated robbery.
On February 17, he was in the exercise yard with the victim, who was participating in a "mutual fight".
Williams punched the prisoner in the back of the head, causing him to fall to the ground, where he lay unconscious for three minutes.
The victim spent the night in hospital, but recovered without long-lasting injuries, the court heard.
Judge David Robinson said the attack was "calculated" and Williams had used a "tremendous amount of force".
"What you did was the definition of what we call a coward’s punch," the judge said.
Williams’ attitude towards the offending was concerning, with the judge noting he laughed when discussing the offending with a probation officer and boasted about the injuries he had caused.
"You seemed pretty pleased with what you’d achieved," Judge Robinson said.
The court heard the victim was a member of an opposing gang.
The judge sentenced Williams to 10 months two weeks’ imprisonment on top of the sentence he was serving.
"You’ve made light of some of the process," the judge said.
"This stuff is serious, really serious. You need to reflect on your choice of associates, your gang allegiance and your attitude toward violence."
