He will be 69 years old by the time he is eligible for parole.

Rappel, and other prisoners serving lengthy sentences for violent crimes, cannot be allowed to think that more of the same can be carried out with impunity or even reduced consequences, the appeal judges said in a decision handed down on Friday.

The 44-year-old was already serving more than 32 years jail for the brutal murder of his ex-partner Tara Costigan while she held their infant. He has since been transferred to Goulburn supermax prison, where he is currently serving his sentence.

In August 2017, he and four other prisoners Alex Bourne, 31, Daniel Grech, 29, Jeremy Leigh Jackson Reid, 27, and Isileli Paea Taholo, 32, were accused of bashing two brothers in their cell at the Alexander Maconochie Centre.

The younger brother was admitted to hospital with 36 individual injuries, including potentially a life-threatening collapsed lung and fractured ribs and facial bones. The elder was also taken to hospital so a tear injury to his ear could be stitched up.

They all denied the charges arising from the beating. But mid-trial Rappel - serving the longest sentence of all accused - changed tack and pleaded guilty to two counts of intentionally inflicting grievous bodily harm, taking responsibility for the injuries caused to the two brothers. The maximum penalty for the charge is 20 years jail.

Prosecutors dropped the charges against the other accused except Grech, who was caught on camera throwing a single punch before the fray moved into the cell and out of sight of surveillance cameras.

The sentencing judge Michael Elkaim had jailed Rappel for 18 months for the assaults, adding an extra three months to his non-parole term.

But the terms prompted prosecutors to appeal immediately after the sentence was handed down on the grounds it was manifestly inadequate.

On Friday an appeal court comprised of Chief Justice Helen Murrell, and Justices David Mossop and Robert Bromwich changed the head sentence to six years, and changed the non-parole term to mean he'll serve an extra three years in jail.

"The conclusion of manifest inadequacy is not difficult to draw in this case," the judges said.

"The sentences were starkly outside the bounds of what was necessary and appropriate. A mere additional six months head sentence, and worse, a mere additional three months non-parole period does not come close to meeting the evidence statutory purpose, nor the broader principles of denunciation, punishment or deterrence both general and specific."

Rappel will be eligible for parole on February 27, 2044.