Richard Jason Reay, 47, was found guilty in a judge-alone trial of strangling Geoffrey Fardell at the Mid North Coast Correction Centre near Kempsey in June 2019.

Justice Robert Allan Hulme sentenced Reay to a total term of 30 years in the NSW Supreme Court on Wednesday.

The judge described it as "an offence involving an unexplained but extreme act of the most brutal violence directed to ending a man's life".

Reay, who has previously been jailed for striking a man's head with a baseball bat for no apparent reason, originally denied any involvement saying he awoke to find the 52-year-old on the floor, telling a floor sweeper "my celly is dead".

At trial he pleaded guilty to manslaughter after a lesion mark was detected on Fardell's neck, claiming a story of self-defence over a television volume dispute.

Reay testified that Fardell menacingly ordered "turn the TV down" before leaping on him and delivering "a flurry of punches," causing him to rip down a clothesline that he'd strung above his bed, wrapping it around his neck.

But Justice Hulme threw out Reay's "entirely implausible account of events" saying his story "makes no sense at all," and found him guilty of murder.

"Isolated is what an inmate is when they're locked in a cell for 15 hours a day with another inmate, not necessarily of their choosing," Justice Hulme said in his sentencing remarks.

"The fact that the victim of an act of violence is an inmate confined in a cell with no practical means of escape, should be regarded as a serious aggravating factor."

The judge afforded Reay no mitigating factors given he was not genuinely remorseful, did not have good prospects of rehabilitation and was likely to re-offend, he said.

Australian Associated Press

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