Aubrey James Agostino launched a violent assault on the prison guard in October last year, punching the man repeatedly in the body and head.

Magistrate David Mossop yesterday jailed the 22-year-old for eight months for the ''significant and sustained assault on a prison officer who was simply doing their job''.

''Prisons are only able to function effectively if prisoners obey the law and the requirements of their incarceration,'' he said.

The defendant, who has previous convictions for assault, was at the time of the incident one month into a six-month jail term for assaulting a police officer.

On the afternoon of October 2, Agostino was being held in the Alexander Maconochie Centre's management unit after threatening prison staff.

The ACT Magistrates Court heard the prisoner threw wet toilet paper at a camera in his cell.

The victim opened the door to clean up the mess and Agostino threw a flurry of punches at him.

Agostino was found guilty of common assault after a contested hearing, with the defendant alleging the guard threw the first punch. But Mr Mossop rejected Agostino's argument and convicted him for an offence ''at the upper end of [the scale] of seriousness''.

In a victim impact statement, the bashed guard described suffering ongoing back pain as a result of the assault.

He also suffered a financial loss because of his inability to take on physically strenuous tasks, and believed he suffered the symptoms of post-traumatic stress disorder.

Prosecutor Anthony Williamson said any sentence had to send a message of denunciation to the community to ensure prison staff were protected while on duty.

The court heard Agostino had a troubled upbringing and was diagnosed with bipolar disorder in 2008.

His lawyer said it appeared the management unit was understaffed at the time of the assault, and his client should not be punished for staffing shortfalls.

But the magistrate said, given the need for deterrence and Agostino's previous failures to comply with good-behaviour orders, a custodial sentence was the only option.

The defendant will be eligible for release in November, taking into account time served.