An internal management review team has reviewed the incident of November 3 last year and the appalling lapse in security which resulted in an emergency lockdown of the nearby detention unit, the first in the prison's 11-year history.

Two incidents occurred at the perimeter fence on the same quiet Sunday summer evening at the Alexander Maconochie Centre last year.

In the first, an unidentified person approached the outer fence and threw over what is described as a "large package".

While this triggered the alarm, no further investigation was made.

"When the alarms sound, the process is we then move the cameras to explore the breach," the Commissioner of ACT Corrective Services, Jon Peach, said.

"On this occasion, that does not appear to have resolved the issue."

Prison officers who would not be identified have disclosed to The Canberra Times that false perimeter alarms are not uncommon due to animals like wallabies moving close to the jail's fence line. One officer disclosed the perimeter alarm goes off "two or three times a week, and usually at night".

However, this time the package of contraband landed between two fences in an area inaccessible to the inmates.

So around four hours later, the same person returned, cut a hole in the fence with boltcutters, retrieved the package and threw it over a secondary fence "Into an area accessible to detainees".

The package was then retrieved and disappeared into the jail, its contents still not fully known

Mr Peach is careful not to disclose what he thinks the large package contained because a subsequent lockdown and search turned up a range of contraband including mobile phones, hand tools, a hacksaw blade, drugs and drug paraphernalia, along with a slingshot and tape.

"I'm as confident as I possibly can be that we got all the contraband that was in the jail," he said.

However, the length of time between when the contraband landed inside the jail to when the breach was detected would have easily allowed the contents to be shifted around and hidden, prison officers say.

The review into the incident found while all physical security systems were operating as normal, more "clarification" was needed for appropriate perimeter checks of the prison.

READ MORE:

It also flagged changes which would make it illegal for a person to be within a certain distance of the jail's secure perimeter.

ACT Corrective Services commissioner Jon Peach said it was disappointing that human error led to the hole not being discovered for 24 hours.

"Full compliance with established processes would have negated the failure to detect a breach of this nature," Mr Peach said.

The review said temporary fences to block the hole were put up the day after the incident.

In response to the November lockdown, enhanced security procedures have been set up at the prison.

However, the exact security upgrades have not been publicly released.

"Further actions in relation to the delays identification and response to this incident will be addressed in accordance with established internal processes," the report said.

Five recommendations were listed by the report to strengthen security at the prison.

The report did note the emergency impact was lessened for families and friends of the detainees due to advance notice provided in the wake of the lockdown.

The lockdown lasted for almost a week with inmates detained in their cells for 23 hours a day while a search for contraband items was carried out.

ACT Corrective Services Minister Shane Rattenbury said work was being undertaken to ensure the incident would not be repeated.

"I thank Mr Peach for his leadership and approach to this incident and his ongoing work to improve practices at the centre," Mr Rattenbury said.

ACT correctional services inspector Neil McAllister said the report into the incident was a frank assessment of what happened.

"I am satisfied that the AMC physical security system was sound at the time of the incident, but there were some response failings which need to be addressed," he said.

"The report makes pertinent recommendations that should be adopted and actioned by ACT Corrective Services."