The hole was cut on Sunday but only discovered on Monday during a routine perimeter patrol, ACT Corrective Services head Jon Peach told a Legislative Assembly committee hearing.

"We then obviously reviewed all the CCTV vision and identified that a member of the community had cut a hole in the fence and had thrown a package in the jail," he said.

"We can't see on the footage exactly how it was done but I would suggest that from looking at the nature of the hole it would have had to have been cut."

The Alexander Maconochie Centre went into a short lockdown on Monday to make sure no prisoners had escaped, then those housed in the area closest to the hole were moved elsewhere and their cells searched.

Mr Peach declared an emergency situation for the jail and placed the whole centre in lockdown for the next few days while searches for the package continue.

"I am not satisfied that we have recovered all the items that may or may not have come through the fence at that point," he said.

"We know of the package being thrown, we haven't been able the retrieve the package."

Under the emergency declaration, prisoners will be forced to stay in their cells for 23 hours a day.

The fence has alarms and cameras trained on it, although it is believed the package was thrown into a CCTV blind spot.

Mr Peach said an alarm did sound when the hole was made on Sunday, and an investigation is now underway to find out "what systems and processes were or were not taken at the time".

ACT opposition corrections spokeswoman Giulia Jones said there was evidence to suggest firearms may have been smuggled into the jail.

"This is a highly disturbing incident. I have grave concerns for the safety of all inside the facility," she said in a statement.

Australian Associated Press