The first case at the clinic, which is run by Ramsay Health Care, was reported to authorities on March 24.
However, The Age has been told some staff were not informed until April 7, while others only found out about the outbreak on Thursday.
Dozens more close contacts of those infected are being contacted by health authorities, while the source of the outbreak is subject to an ongoing investigation.
Ms Mikakos said all patients and staff at risk were being tested and no new patients would be accepted at the clinic, which is undergoing deep cleaning.
An email from Victoria’s Chief Psychiatrist Neil Coventry to mental health services across the state on Thursday, a copy of which has been seen by The Age, urged staff to self-quarantine and monitor themselves for symptoms.
In the email, Dr Coventry said any person who attended the clinic “on or after March 24” was considered at risk of developing COVID-19 and must quarantine for 14 days after their most recent contact with the clinic and undergo testing for the virus.
This includes clinical and non-clinical staff members, patients and visitors.
“The department is investigating an outbreak of COVID-19 amongst patients and staff at Albert Road Clinic,” the email said, adding that it is “actively working with Ramsay Health and the clinic to ensure appropriate contact tracing and public health actions are in place.”
The email said patients currently in hospital would be moved to private rooms, tested for the virus and screened for symptoms daily.
Staff who have attended the clinic have also been ordered not to work in any clinical setting for 14 days after their most recent contact with the clinic.
“This is a precautionary approach until a full investigation can be completed,” the email said. “Individual staff may be contacted by the department to further refine their level of risk and determine requirements for quarantine.”
Albert Road Clinic chief executive Jane Pickworth said the safety of patients, staff and doctors was of “paramount importance” and all non-essential visitors had been banned.
She said the clinic was working closely with the Victorian Health Department regarding a number of staff and recently discharged patients from the clinic who have tested positive for COVID-19. The clinic is also contacting all recently discharged patients, she said in a statement.
Chief Health Officer Brett Sutton said the clinic was being “effectively shut down” and was considered high risk, with patients likely to have chronic illness or other vulnerabilities.
Professor Sutton said the first case at the clinic was reported in late March, but more cases only emerged over the last week, causing concern about ongoing transmission within the facility.
“There has been really significant shutdown of activities in that facility, no new in-patients, no transfers out, no out-patient activity, no group therapy,” Professor Sutton said.
“We are doing very substantial things, but we understand the explosive potential in enclosed facilities as we have seen in hospitals and aged care facilities and other settings like that.”
There had been no new cases of community transmission in Victoria overnight, Ms Mikakos said, with the number of those cases stable at 135.
Twenty-seven people remain in hospital, including 11 people in intensive care. There have been 1254 recoveries in Victoria and more than 96,000 tests.
Ms Mikakos said the cluster was “a warning sign”.
“Today is a demonstration that you can get explosive outbreaks and sometimes these spill out into the community,” she said. “We’re seeing the fragility of the situation. This is not all over yet.”
Melissa Cunningham is The Age’s health reporter.
Aimee Amiga is a journalist at The Age. Previously, she worked in the Middle East at Haaretz-International New York Times and the Jerusalem Post.