Bradley Bedford, 20, was this week jailed for six years over the New Year's Day assault, which the court heard erupted after a verbal argument.

"The victim was a first-time prisoner in the [Alexander Maconochie Centre]. The offender was a seasoned inmate in correctional institutions," Justice Michael Elkaim said in sentencing Bedford.

"He took advantage of his cell-mate's position and beat him ferociously."

Justice Elkaim also set a non-parole period of three years and seven months. Bedford will be eligible for parole in July 2022.

Bedford had pleaded guilty to a charge of recklessly inflicting grievous bodily harm, but was also sentenced in the ACT Supreme Court for various breach offences.

Justice Elkaim noted that the charge for which Bedford had been placed in custody a couple of months earlier did not proceed, and it appeared he should not have been in custody at all.

"This is not a factor of mitigation but, taken together with issues the offender was having in respect of his mental health medication, the frustration that he was experiencing does to some degree place his offending actions into context," he said.

Bedford was sharing a cell with the victim on January 1 this year when an argument erupted between them and devolved into a physical fight. They had appeared to share a cordial relationship, the court heard, and the victim would often provide food and cigarettes to Bedford.

The court heard the fight was one-sided and the victim was severely injured, including a traumatic brain injury, a left carotid artery dissection, a large laceration that will result in permanent scarring and a fractured nose. He spent time in hospital and there were complications with his injuries.

A neuropsychological report made reference to the victim's "very severe impairment of memory and learning on examination", the need for a craniotomy to relieve pressure on the brain and a severe impairment of auditory-verbal and visual memory.

The neuropsychologist also found very slow processing speed and a poor attention and concentration span, and said the man would have difficulty managing his own affairs and require sustained psychological treatment.

"In short the victim is now a severely brain damaged person in his 40s who will have to negotiate the rest of his life with the disabilities that the offender inflicted upon him," Justice Elkaim said.

A reference from Bedford's mother described a difficult upbringing exposed to his father's aggression and abuse. She said he had made a turn for the better and was in stable employment until a "false allegation put him into AMC".

She described him as a "very caring and loving person and is dedicated to and loves his family and friends. Brad can be relied upon and is a hard worker and I know he will find employment".

Justice Elkaim said, "These are all very admirable sentiments but the fact remains that he unnecessarily and viciously beat up another man who now has permanent brain damage."

However the judge continued: "If his mother's assessment is correct, then rehabilitation of the offender must be taken into account. He is still very young and is hopefully capable of putting a violent life behind him."

The ACT Inspector of Corrective Services prepared a critical incident report of the incident published earlier this year. It recommended policies be reviewed to require a risk assessment take place and be appropriately documented for every accommodation placement decision.