A female aged care worker at a facility in Melbourne's northwest is among the five new locally-acquired COVID-19 cases recorded in Victoria.
The woman worked at Arcare Maidstone and may have worked on two days - May 26 and 27 - while infectious, according to authorities. The facility has been placed into lockdown and residents are self-isolating.
Her case is currently unlinked to other COVID-19 cases.
The Victorian Department of Health's Jeroen Weimar told reporters on Sunday that the woman - who is a healthcare provider in contact with aged care residents - had received one COVID-19 vaccination.
She sought virus testing as soon as she experienced respiratory symptoms.
Unlike in the hotel quarantine sector, there is no current requirement for aged care workers in Victoria to get vaccinated against COVID-19.
Some 110 staff work at Arcare Maidstone.
Mr Weimar also said the state government had no access to data on the progress of the vaccine rollout in private residential aged care.
"We know of course that anybody who has family in aged care will be particularly concerned - we will be working closely with them," he said.
"I'm concerned at this point we don't have an original acquisition source.
"My understanding is at this particular site half the residents were vaccinated and around a third of the staff were vaccinated."
Arcare aged care, in a letter to resident families on Sunday, said the Maidstone facility had enough protective equipment to maintain patient support. The providers said they were "well prepared" for the lockdown.
Of the other four local COVID-19 cases diagnosed on Saturday, two were close virus contacts in quarantine and one is linked to a previously-announced exposure site. The final case is also under investigation.
More than 17,700 vaccinations were administered on Saturday, while the five local COVID-19 cases came from more than 45,000 tests.
Most infections are currently linked to Melbourne's City of Whittlesea cluster, with cases testing positive for the B1617 strain first identified in India.
Acting Premier James Merlino on Sunday again declined to comment on the prospect of a lockdown extension, saying no decision has yet been made and no threshold exists to trigger such an extension.
Masks are mandatory and residents can only leave home to shop for essentials, provide or receive care, exercise, work or study or to get vaccinated.
These settings will remain in place until at least Friday.
"The number of cases, the type of cases, whether they are mystery cases, the number of exposure sites, the pace of transmission ... all of those things are taken into account. We are still in early days," Mr Merlino said.
"This is something that is evaluated every hour of every day."
There are currently 49 active COVID-19 cases in Victoria, with authorities uncovering 4000 primary close contacts to date. Mr Weimar said about 70 per cent of those people have tested negative to the virus so far.
Meanwhile, scores of Melburnians queuing for the COVID-19 jab on Saturday faced long waits after technical issues crippled booking systems.
It comes after the state's coronavirus hotline was flooded with more than 77,000 calls in 15 minutes when it was announced on Thursday that eligibility for the Pfizer vaccine would be expanded to people aged 40-49.
There were multiple injuries and 14 arrests when about 150 people gathered in the CBD on Saturday to protest against the lockdown restrictions.
Victorian Treasurer Tim Pallas on Sunday announced a $250 million business support package for the state's fourth lockdown, including $190 million for small and medium firms and $40 million for hospitality venues.
Mr Pallas said the federal government declined to contribute.
