An aged care facility in Melbourne's west is on lockdown after a staffer tested positive for Covid-19, with no known contact link.
The staffer wore a mask when they last worked on May 26 and May 27 but did not have any symptoms until the next day and later tested positive.
Arcare Maidstone said in a statement that no other infections have been found and all residents will now self-isolate in their suites.
Staffers have reduced their movements and will wear full PPE equipment.
"I want to assure you that, whist we hoped that this would not happen again, we are well prepared, and our infection control practices put us in a good place to manage this outbreak effectively," the statement reads.
The state's head of testing, Jeroen Weimar, said on Sunday that finding the source of the aged care case was an "extreme concern" and of the "upmost priority to us".
The staffer was potentially infectious on May 26 and May 27, while she was at work, but had received the first dose of her Covid vaccine, he added.
On-site testing is underway and anyone who was at the facility on those dates is encouraged to come forward.
Hunt for source of mystery Covid cases
Speaking to reporters on Sunday, Victorian deputy premier James Merlino said authorities were still tracking down the source of the aged care case, as well as another case recorded in the past 24-hour period.
“Yesterday we recorded five new locally acquired cases. Two were previously identified primary close contacts and have been quarantining throughout their infectious period," he said.
"One is a link to an exposure site and investigation is under way to confirm the nature of the two remaining. One is a staff member at an Arcare aged care facility.”
He warned there have been more exposure sites added in the last 24 hours however "70 per cent of our primary close contacts identified as of yesterday have retained a negative result".
He added: "But it is true to say that this remains a day by day proposition."
The southern state also diagnosed five new local Covid-19 cases on Friday, four of which were close contacts of a food delivery driver who travelled across north and southeast Melbourne while potentially infectious.
Most infections are linked to Melbourne's City of Whittlesea cluster, with cases testing positive for the B1617 strain first identified in India.
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