Prisoners remain in lockdown at the Alexander Maconochie Centre while corrective services officers search for contraband suspected to have been thrown over the fence into the jail.
A state of emergency was declared at the jail on Wednesday, the first in the prison's 11-year history, after contraband was smuggled in.
ACT Corrections Minister Shane Rattenbury said the suspected illegal items were thrown over the jail's fence after a hole was cut in the prison's outer perimeter fence.
A full search of the jail is being carried out by corrective service officers.
The state of emergency was declared at 2pm on Wednesday and is likely to continue into Friday.
Mr Rattenbury said drug-related contraband had been found inside the prison, but it was not clear if it was part of the parcel that sparked the emergency.
"The lockdown will end when the executive director is satisfied there has been an absolute thorough search," Mr Rattenbury told ABC radio on Thursday.
"They're going through the jail from top to bottom and will continue for another day or so."
Sniffer dogs from NSW and federal police officers have been brought in to help with the search for the contraband in the prison.
One federal police officer was refused entry into the prison after not wanting to surrender their gun to prison staff, citing personal safety.
"An assessment was made where [corrective services] did not want firearms in the jail, given the uncertainty," Mr Rattenbury said.
"The executive director took the decision that they did not want firearms in the jail at the time."
The ACT Opposition has called for a review of security at the Alexander Macononchie Centre in the wake of the incident.
Canberra Liberals spokeswoman for corrections, Giulia Jones, said there major concerns that the hole cut in the fence on Sunday evening was only discovered the following day.
"In this current era, we must have good enough security to see the perimeter fence, when we know there's a problem with items being brought in," Ms Jones said.
"We've been told the cameras that monitor the fence move and don't stay static. [Security] hasn't worked in this instance."
Prisoners will remain in their cells for 23 hours a day while the lockdown is in effect.
All prisoners have been accounted for.
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.
Ms Jones said there were grave concerns over the contents of the contraband package, saying it posed a risk to staff at the prison and detainees.
"This is not good enough for the families whose family members come and work here, and it's not good enough for the detainees who are trying to do the right thing," she said.
"This is putting enormous pressure on staff."
Mr Rattenbury said 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.
Authorities are yet to determine the identity of the person who cut the hole in the fence and smuggled the items into the prison.
"There are a range of cameras and alarms and we're going back through the details," Mr Rattenbury said.
"Undoubtedly there will be some lessons that will be learnt from this process."
The minister said a review would be conducted into how the contraband managed to get into the jail.
"Possibly this is an extreme and desperate way to get contraband into the jail," Mr Rattenbury said.
"An internal management review will happen once the matter has been identified and work is still ongoing."
The opposition spokeswoman said there were also concerns around the welfare of detainees, who were not given access to medical treatment in the early hours of the lockdown.
"Dosing has resumed [on Thursday], however detainees are allegedly being told they cannot see a doctor unless they are in a life or death situation," Ms Jones said.
"What this means is that if any detainees are experiencing side effects from their resumed medications, which were unfortunately stopped on Wednesday, they do not have access to important medical device."
Briefings have not been provided on the ongoing situation to the ACT opposition.
The Community and Public Sector Union, which represents the prison staff, expressed "serious concerns for the safety of our members".
Regional secretary Maddy Northam said the union had "numerous reports that there are weapons, potentially firearms, in the AMC" and urged the prison executive to accept an offer by police to assist with the search.
"CPSU members want to work with WorksafeACT and AMC management to make sure there is a safe outcome to this unfolding situation," she said.