A court has been told a 25-year-old man sentenced for an assault on a fellow Hawke's Bay Prison inmate faces a lifetime in jail if he doesn't change his ways.

In Napier District Court on Friday, Judge Geoff Rea said: "If he stops whacking people he'll be right."

Defence counsel Leo Lafferty was concerned about the time defendant Tyson Tamati Te Tomo will have spent in jail with the extension of his incarceration as a result of a sentence for injuring with intent to injure in the incident which happened at Hawke's Bay Prison on October 20 last year.

Te Tomo went to another inmate's cell, blocked the exit, and began an attack in which another inmate joined soon afterwards. The victim suffered facial injuries and fractured ribs.

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Mr Lafferty calculated that with the expected sentence Te Tomo will have been in jail for 5 years and 9 months continuously by due release date.

He wanted steps in place to help Te Tomo complete the sentence without further "misconduct," and lessen the chances of reoffending.

"You have a real prospect of becoming institutionalised," Judge Rea told Te Tomo. "If you keep on doing it you are going risk a sentence of preventive detention before you are very much older."

Te Tomo was sentenced to a further 2 years and 5 months in prison.