Harrowing details of the January incident, rated as critical and requiring assessment by the service's independent inspector, were submitted to the ACT Assembly on Wednesday.

A number of issues were highlighted in the report, both before the New Year's Day bashing and immediately afterward, when the 20-year-old perpetrator was placed into solitary confinement on "investigative segregation" for an extraordinary 27 days while the police investigation continued.

The inspector found that cells at the overcrowded Canberra jail were subjected to the customary "lunch-time lock-in" when the call for help came from the cell's intercom.

A "code pink" was called and five corrections officers attended.

The offender was taken away, put in solitary and later charged with assault occasioning grievous bodily harm while the victim was initially treated at the jail's health centre but his condition deteriorated so badly in the following 36 hours that he required emergency brain surgery two days later.

After almost a month in hospital, the 40-year-old is now out on bail, but still suffering the after-effects of a subdural haemorrhage.

The reasons for the assault remain unclear, although the report suggested it may have been a dispute about "smokes".

The victim in this incident had been serving a short sentence for an undisclosed violent offence and had only been in the jail for a few weeks when the bashing occurred. When brought to the jail, he had expressed his anxiety to staff about it being his first time in custody.

The perpetrator in the incident had a history of mental health issues, together with a record of serious and violent offences which began with youth detention.

As the pressure on the prison population grows, the double-bunking of inmates, the mixing of remandees and sentenced prisoners, and the lack of transparency around the decisions to place detainees in a shared cell, raise ongoing issues for prison management.

In February this year, the ACT Ombudsman reported how the amount of time spent by Canberra inmates locked in their cells was one of the biggest causes of prisoner complaint.

The inspector's report into the January incident found that while both prisoners sharing the cell were classified as "medium" security, there was "no record of the reasons why they were placed in a shared cell or that appropriate factors were considered".

The review noted that there were no relevant notes or "flags" placed on either the victim or the offender to indicate the incident was "reasonably foreseeable", and found no failings of security procedures or practices.

Corrections Minister Shane Rattenbury offered no comment on the report, and said the government would "respond in November".

ACT Liberals' spokesperson Guilia Jones expressed her concern that "this is yet another report where the lack of appropriate record-keeping is disturbing".

"I have been told by so many corrections officers that there's no record-keeping about how prisoners are assessed about their suitability to share a cell," she said.

"We can only assume that's a direct result of the overcrowding issue."

The inspector of correctional services, Neil McAllister, has an extensive review into the "health" of the ACT prison due in October.