The killer of Canberra woman Tara Costigan could remain in jail until he is 78 years old, after a Canberra court substantially increased his sentence for an attack on two brothers in prison.

Key points: Rappel is serving a 32-year sentence for murdering Tara Costigan

Rappel is serving a 32-year sentence for murdering Tara Costigan His sentence was initially extended by six months over a prison bashing

His sentence was initially extended by six months over a prison bashing On appeal the six-month extension was increased to six years

Marcus Rappel, 44, was already serving more than 32 years for murdering Ms Costigan, his former partner, with an axe as she fled with their week-old baby in her arms.

Last year an extra six months was added to his total sentence, with his non-parole period extended by three months for the prison assaults, which left one of the victims in a potentially life-threatening condition.

Rappel killed Tara Costigan with an axe while she held their newborn. ( Photo: Facebook )

Today the ACT Court of Appeal issued a stern rebuke to the initial sentence, increasing it from an effective extension of six months to six years.

"The sentences imposed were starkly out of the bounds of what was necessary and appropriate," the judgement said.

"Mr Rappel, as well as other offenders serving lengthy sentences for crimes of violence, cannot be allowed to think that more of the same can be carried out with impunity, or even with reduced consequences."

Rappel and several others were charged with an attack inside Canberra's jail in 2017, when the two brothers were badly hurt and taken to hospital.

One of the men had 36 individual injuries with cuts and bruises to his face, chest, torso and knee, and a fractured eye socket and broken ribs.

His brother suffered head and chest injuries, including a torn ear.

Two days into a trial over the matter Rappel pleaded guilty, and charges were dropped against most of the others.

But when he was given only six months more in jail, prosecutors took the case to the Court of Appeal saying it was manifestly inadequate.

Rappel will also serve another three years non parole, which now runs out in February 2044.