A bashed prisoner, later placed in an induced coma due to his severe injuries, was unattended for 45 minutes before he was discovered by Alexander Maconochie Centre staff.
Prison staff failed to conduct routine patrols and allowed prisoners to cover cell door windows with towels, meaning they had "no idea" what was going on inside, a report from the ACT's inspector of correctional services found.
It said these failings contributed to the delays in his severe injuries being discovered.
The assaulted prisoner, David Christopher Laipato, has a lengthy criminal history and has been in the prison since January last year for violently assaulting the mother of his four-year-old child.
In a "bizare and unfortunate" series of events, Laipato's father only found out about the brutal bashing when he saw his son being taken to the intensive care unit at Canberra Hospital, when he happened to be visiting his wife.
But the report, tabled in the ACT Legislative Assembly, found staff technically interpreted the legislation correctly in determining not to immediately contact next of kin.
The report said Laipato was in his unlocked cell on January 13 when three other detainees entered and closed the door behind them.
CCTV showed a number of prisoners pacing up and down the area outside the cell and looking into it over the next 15 minutes.
About 45 minutes after the attack began, the Laipato was helped to a common room, where he sat in a groggy state with a lot of blood around his head. It was then staff were alerted to his injuries.
Laipato suffered multiple lacerations to his head and ankle, and bruising and fractures to his nose and throat.
The throat injury resulted in him being placed in an induced coma at Canberra Hospital, the report said.
The inspector's report said he was concerned to see, after viewing several hours of CCTV vision, hardly a staff member was seen in the area of the unit where the assault took place.
"We also noted that every cell visible on the primary camera had its observation window covered by a towel, usually with the door closed," the report said.
"Put simply, staff would have no idea what is going on in those cells unless they opened every door."