An internal investigation has been ordered at Canberra's prison after a hole likely used to smuggle contraband in for prisoners was found in the outer perimeter fence.
ACT Corrective Services confirmed on Tuesday that a breach had been located in the jail's perimeter fence.
It would not disclose where in the fence, or in which direction the breach was located.
However, an impromptu roll call revealed no prisoners had escaped as a result of the breach and Corrective Services is confident "no detainees breached the perimeter at any time".
ACT Corrective Services confirmed that "investigations show a parcel was thrown onto the prison grounds by a community member".
Designated as a maximum security prison, the Alexander Maconochie Centre at Symonston has three sets of security fences.
The breach in the outer fence, which occurred around three days ago, has allowed someone to gain entry to the inner ring of fencing on one or possibly more occasions.
Downward-facing spikes are located on the inner fence to discourage climbing but conversely, any items thrown from within the "no man's land" between the fences are likely to slide down the steel bars and directly into the inner grounds of the prison.
"All detainees located in the area close to the perimeter breach have been searched and relocated to other accommodation within [the prison]," a spokesman for Corrections Minister Shane Rattenbury said.
"The accommodation unit in question is currently being searched thoroughly."
The issue of contraband items being thrown into the outer yard has been an irregular occurrence for years, as a health and welfare survey found more than 89 per cent of people entering Canberra's prison are previous illicit drug users.
In 2014, a 19-year-old man was arrested after throwing a package of amphetamines over the fence.
Preventing this occurring has been the source of robust discussion, with Corrective Services maintaining that it is a police responsibility to patrol those areas outside the jail perimeter.