Prison guards are scouring Canberra's jail cells for potential guns after a hole was discovered in a perimeter fence earlier this week.

Key points: Canberra's Alexander Maconochie Centre is being searched for contraband, which may include guns

Canberra's Alexander Maconochie Centre is being searched for contraband, which may include guns A hole was found in a perimeter fence earlier this week but an emergency roll call had all detainees accounted for

A hole was found in a perimeter fence earlier this week but an emergency roll call had all detainees accounted for The union representing prison guards says authorities have requested a weapons detection dog from NSW

Authorities believe an unidentified person cut the hole, climbed through it and tossed a package of "contraband" over two internal fences into the jail.

The Alexander Maconochie Centre (AMC) remains in an official state of emergency — declared yesterday for the first time in the prison's history — and the ACT Government has launched an internal review of the incident.

Drug-related items have already been found but authorities said they were unsure if they were from the package smuggled inside. They have also not confirmed whether the jail was being searched specifically for guns.

The emergency lockdown, which is expected to go on for days, means inmates are confined to their cells for 23 hours a day with only an hours break for physical exercise.

The Community and Public Sector Union (CPSU), which represents the prison guards, said it had serious concerns for the safety of its members working at the AMC.

"We've had numerous reports that there are weapons, potentially firearms, in the AMC and this obviously poses a significant risk to staff and other detainees," regional secretary Madeline Northam said.

The lockdown was initiated on the same day Australian Federal Police officers and a firearms dog were called onsite to provide expert help but turned away when they refused to enter the jail unarmed, as per AFP policy.

ACT Corrective Services Minister Shane Rattenbury said the decision was made for safety reasons.

"The executive director took a decision that he did not want firearms inside the jail at this time," Mr Rattenbury told ABC Radio Canberra.

The minister denied there was a safety policy conflict between the jail and the AFP.

"This is a discussion that continues based on the matters at hand and so at different times different decisions will be made based on the circumstances," he said.

A hole was found in the perimeter fence earlier this week. ( ABC News: Jonathon Gul )

He said authorities were concerned "the contents of the parcel have not been fully recovered."

"The lockdown will end when the executive director John Peach is satisfied that there has been an absolutely thorough search of the jail," Mr Rattenbury said.

"The material that has been found has been drug-related contraband."

NSW weapons detection dogs requested, CPSU says

The CPSU said the AMC had requested a weapons detection dog from New South Wales Corrective Services.

In the meantime, correctives services staff have been searching for potential weapons inside the jail without the support of highly-trained police.

"The AFP are the professionals in this space," Ms Northam said.

"We would expect that AMC senior management would be relying heavily on the AFP, not turning them away."

The Opposition said it was disgraceful that security cameras at AMC failed to identify who cut a hole in the outer fence and they have called for changes to way the prison is run.

"The head of Correctional Services said yesterday that he was surprised to find that they didn't see this on a video and I think that's a disgrace," Canberra Liberals spokeswoman Giulia Jones said.

"I think in the current era we must have security good enough to view the perimeter fence at all times."