A prisoner who headbutted a guard at a Canberra jail will spend more time behind bars.

Aubrey James Agostino has been convicted of assaulting staff at the Alexander Maconochie Centre for a second time.

Agostino pleaded guilty in the ACT Supreme Court to assault occasioning actual bodily harm.

Justice John Burns on Tuesday sentenced Agostino to 20 months' jail, with a non-parole period of one year.

The sentence effectively adds another year to Agostino's existing sentence, which expires next month

The court heard the 23-year-old attacked the guard during an internal transfer in December 2011.

Agostino became upset when told changes would be made to his prisoner management plan.

He refused to wear handcuffs during the transfer and attempted to push past officers, who then restrained him.

In the ensuing struggle Agostino called the victim a "f-----g dog" and headbutted him. The blow broke the guard's nose, causing bleeding, and left a 1.5-centimetre laceration.

The guard underwent surgery twice to correct the damage, but has been left with a permanent breathing disability. The court heard the guard was off work for nine weeks and underwent counselling as a result of the assault.

Agostino had previously been found guilty of a similar offence of violence against prison staff.

In April last year the sentenced prisoner was found guilty of common assault by the ACT Magistrates Court for bashing a guard in October 2011. On that occasion, Agostino 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, striking him in the body and head.

The victim suffered ongoing back pain and symptoms of post-traumatic stress disorder.

Agostino will be eligible for parole next March.