First-time inmate Minh Dang, then 41, had been behind bars for less than two weeks when cellmate Bradley Bedford, already a veteran of the correctional system at just 20, bashed him on New Year's Day in 2019.

The Vietnamese man was granted bail a week or so later and has not returned to jail since, while Bedford was sentenced to six years for the attack.

During his time on conditional liberty, Dang launched an appeal against the severity of his prison term.

Chief Magistrate Lorraine Walker had sentenced him to imprisonment for 15 months over the drunken and apparently unprovoked March 2017 glassing of a stranger at a Canberra bar.

Agreed facts tendered at Dang's sentencing revealed he had smashed a glass into the side of his victim's head, cutting the man behind his left ear and leaving him bleeding profusely.

He pleaded guilty to a charge of assault occasioning actual bodily harm, and his jail term was to be suspended after three months.

Dang's barrister, Jack Tyler-Stott, argued on appeal last December that Ms Walker made an error by taking into account that the glassing victim had apparently been left with permanent scarring.

He said this had not been part of the charge to which Dang pleaded guilty, nor had it been referred to in the agreed statement of facts before the chief magistrate.

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Mr Tyler-Stott claimed Dang had effectively been punished for a more serious offence, involving grievous bodily harm, for which he had not been convicted.

Associate Justice Verity McWilliam, who heard the appeal in the ACT Supreme Court, agreed on Thursday.

"The sentencing court has inadvertently, but impermissibly, taken account of conduct that could have been separately charged," she wrote in her judgment.

Associate Justice McWilliam was therefore required to resentence Dang, whose "vicious" prison bashing had left him briefly comatose and with serious mental impairments.

She said he was unlikely to ever regain his full cognitive capabilities, meaning the sentencing principle of specific deterrence had "all but evaporated".

Dang, who has been unable to work since he was attacked, had also been assessed as now being an extremely low risk of reoffending.

Associate Justice McWilliam ultimately found a jail sentence was still appropriate for Dang, though she said it should be shorter than the one originally imposed by Ms Walker.

Taking into account that he had already spent time in custody and that a return to the Alexander Maconochie Centre may worsen Dang's psychological state, she resentenced him to a fully suspended term of 10 months in jail.

The judge also ruled that Dang still had to pay the glassing victim $918 to cover the cost of an ambulance trip, as originally ordered by Ms Walker.

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