The attack occurred while Aleksander Vojneski, 31, was in the Alexander Maconochie Centre in August last year, waiting to go on trial for stabbing his girlfriend Paula Conlon to death.

Vojneski, a man with serious mental health issues, believed someone had forged his name on a "buy up", which allows prisoners to use their own money to buy groceries and other goods.

Believing his own money had been used by someone else, he asked a guard for help.

The guard told Vojneski he would investigate, but the prisoner became angry at his response.

The prison officer walked out into a yard, and CCTV vision showed Vojneski rushed out sometime later.

Vojneski walked straight up to the guard and punched him three times in the head.

Other guards rushed over and pulled Vojneski off the guard.

The ACT Magistrates Court sentenced Vojneski to 12 months imprisonment on Friday morning, after he pleaded guilty before Chief Magistrate Lorraine Walker.

The sentence will run concurrently to the life sentence he received earlier this month for the brutal murder of Ms Conlon.

The 30-year-old mother of three was stabbed to death in the bedroom of her Macgregor home in 2012, after Vojneski became frustrated by failed attempts to get the drug ice on credit.

The sentence for the prison guard attack will have little practical value, because it will be served at the same time as his life sentence for murder.

But Ms Walker said a strong message needed to be sent to other inmates that they could not get away with criminality just because they were serving lengthy sentences.

She also gave Vojneski no discount for his guilty plea, saying it had little practical value because it came so late, did not reflect any remorse, and was made after extensive delays.

Prosecutor Joel Hiscox said the assault was aggravated by the fact it occurred in custody and against a guard who was just doing his job.

"Prisoners cannot engage in this conduct, the community does not accept it," he said.

He said the assault was "completely disproportionate" to any disagreement he may have had with the guard.

Vojneski's defence lawyer Peter Woodhouse said his client was entitled to a discount for his guilty plea, and said the common assault was not the most serious example of an offence of its type.

Mr Woodhouse said the assault was very brief and "spur of the moment".

He said there were special circumstances - including the fact that Vojneski was serving life - that meant the sentence should not be served on top of the existing term of imprisonment.

Vojneski is only eligible to be freed from prison after application for release on licence. Any decision to release Vojneski on licence would be made by the ACT government.

Ms Conlon's mother and close friend were in court to watch Friday's sentencing.

Vojneski's family, present throughout his murder proceedings, were absent.

The killer has appealed against his life sentence for the stabbing murder, arguing the sentence was manifestly excessive.