The Canberra jail emergency and lockdown will continue until next week as staff search for contraband, which possibly may include a firearm, thrown into the prison yard earlier this week.
The initial state of emergency was declared at 2pm on Wednesday.
"It is essential that we undertake a full and systematic search of the AMC to satisfy any risk that may have been presented through the introduction of this parcel," a spokesperson for the Justice and Community Safety directorate said.
The executive director of ACT Corrective Services, Jon Peach, confirmed the extension of the initial three-day declaration under Section 26 of the Corrections Management Act of 2007, "effective immediately".
Under the Act, an emergency may be declared within a correctional centre for a stated period of not more than three days. A declaration may be made for two or more consecutive periods in relation to the same emergency.
"This will see the powers under the declaration continued until 1400 hrs on Tuesday," the spokesperson said.
"No further statements will be made at this time. However, a comprehensive media brief will be provided once the operation concludes," the spokesperson said.
Prisoners will remain in their cells for 23 hours a day while the lockdown is in effect.
A hole was cut out in the outer perimeter fence that was big enough for a person to get through before the parcel was thrown over the fence earlier this week.
While the parcel and its undisclosed contents were retrieved by staff from the yard, grave concerns remained that other items of contraband may have been taken into the prison before the parcel was discovered.
Authorities are yet to determine the identity of the person who cut the hole in the fence.
While security cameras monitor the "sterile" area between the fences, they are not static and the breach remained undetected until staff doing a perimeter check found the hole on Monday at 5pm.
Sniffer dogs from NSW and federal police officers have been brought in to help with the search.
One federal police officer was refused entry into the prison after not wanting to surrender their gun to prison staff, citing personal safety.
All visitation to the jail has been cancelled while the lockdown is in place, but prisoners still have access to health appointments and can appear before court if they are scheduled.
An internal management review will happen once the emergency is over.
The Community and Public Sector Union, which represents the prison staff, expressed "serious concerns for the safety of our members".
The ACT Opposition has called for a review of security at the Alexander Macononchie Centre in the wake of the incident.