Mervyn Keith Davidson was on remand for several violent offences in February 2018, including a vicious attack in which an Albion Park Rail Aldi worker was knocked unconscious, when he walked into 54-year-old Alfredo Pengue's cell at Silverwater prison, shut the door and strangled him.

Davidson had taken five Xanax and smoked "ice" before the horrific and "prolonged" jailhouse assault, of which he later claimed he had no memory.

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Davidson was charged with murder but acquitted after a judge-alone trial in August 2019 and instead found guilty of manslaughter. The court heard Mr Pengue's lifeless body was discovered by another inmate shortly after Davidson left the cell.

He had fractures to multiple ribs, extensive bruising to the neck and head area and blood was dripping from his mouth.

Meanwhile, Davidson was calm and composed when interviewed by police but refused to answer questions.

"I know what I'm here for. Just hurry up and do the charge paperwork so I can go back to my cell and sleep," he said.

The court heard at the time of the killing, Davidson was awaiting sentence for attacking a female Aldi worker with a baseball bat during a botched supermarket robbery in Albion Park Rail in January 2017.

While in custody for that matter, he punched a police officer and choked his Silverwater cellmate with a t-shirt until the man became unconscious.

In March 2018, Judge Andrew Haesler sentenced Davidson to 11 years' jail over the Aldi assault and first inmate choking.

He set a non-parole period of 7 years and 8 months, meaning with time already served, Davidson would become eligible to apply for parole in September 2024.

However, Supreme Court Justice David Davies last week extended Davidson's minimum time behind bars by a further five years for Mr Pengue's manslaughter.

He imposed a head sentence of 11 years' jail, with a non-parole period of 7 years, but agreed to partially overlap it with his current sentence.

He noted psychologists had determined that Davidson presented a "moderate to high risk of reoffending" and that his prospects of rehabilitation were guarded.

"The offender viciously attacked two other prisoners within a 12-month period," he said.

"In terms of his rehabilitation after any release on parole, I hold considerable reservations."