A Melbourne aged care resident and another worker have tested positive for COVID-19 as authorities brace for the outbreak to get worse before it gets better.
Health Minister Martin Foley confirmed a further six locally acquired cases were detected past the midnight cut-off, on top of the five already reported on Monday.
On the fourth day of Victoria's seven-day lockdown, Acting Premier James Merlino said authorities were running "neck and neck" with the virus.
"There is no doubt, the situation is incredibly serious," he told reporters.
"The next few days remain critical. I want to be very clear with everyone, this outbreak may well get worse before it gets better."
Health authorities have identified 4200 primary close contacts, with 77 per cent of those returning negative tests so far.
Victoria's Chief Health Officer Brett Sutton said a number of those primary close contacts were based in regional Victoria.
Mr Merlino said it was too early to tell if the statewide lockdown would need to be extended beyond 11.59pm on Thursday, saying it was a day-by-day proposition.
"We've got a lot of exposure sites. We've got a lot of high-risk sites that we are concerned about," he said.
Three of the 11 new cases reported on Monday are linked to an outbreak at Arcare Maidstone in the city's northwest, after a healthcare worker at the facility returned a positive result on Sunday.
An Arcare Maidstone resident, a woman in her 90s, is among them and has since been moved to hospital.
"The fact this outbreak has crept into a number of private residential aged care facilities is obviously of very great concern to the Victorian government," Mr Foley said.
The Altona woman's son and an unvaccinated colleague, who also worked at BlueCross Western Gardens in Sunshine three days last week, have also tested positive.
More than 600 people living in aged care homes died last year as a result of Victoria's devastating second wave of COVID-19, which lead to a 112-day lockdown.
The majority contracted the virus from staff, many of whom were working across multiple facilities.
Mr Foley said he would resist the temptation to criticise the federal government, after it repealed rules banning private aged care staff from working across multiple sites.
Contact tracers are yet to determine how the first worker caught the virus and hope genomic sequencing, expected back within the next 24 hours, can link the case to the original City of Whittlesea outbreak.
Unlike the first worker who had received her first dose, her colleague had not received her first COVID-19 jab as she was "on personal leave" when vaccinations took place.
Arcare chief executive Colin Singh said the elderly resident had received one dose of the Pfizer vaccine and was awaiting a second dose.
Only a third of Arcare Maidstone's 110 staff and 53 of 76 residents have been vaccinated so far, with more expected to receive their first or second dose on Monday.
Health Minister Greg Hunt said 85 per cent of residents in private aged care facilities and 100 per cent in Victorian residential facilities have been vaccinated.
Australian Nursing and Midwifery Federation Victorian branch secretary Lisa Fitzpatrick said vaccination rates at the Maidstone facility demonstrated Canberra's "go-slow culture" on aged care.
The Blue Cross Western Gardens facility as well as Royal Freemasons' Coppin and Footscray sites have joined Arcare Maidstone in locking down residents in their rooms.
