A Melbourne aged care resident and another worker have tested positive for COVID-19 as authorities brace for Victoria's outbreak to get worse before it gets better.
Acting Premier James Merlino confirmed a further six locally acquired cases were detected past the midnight cut-off, in addition to the five already reported on Monday.
On the fourth day of Victoria's seven-day lockdown, Mr Merlino said a small cohort of cases were infecting a large number of contacts.
"There is no doubt, the situation is incredibly serious. The next few days remain critical," he told reporters.
"I want to be very clear with everyone, this outbreak may well get worse before it gets better."
Mr Merlino added it was too early to tell if the statewide lockdown would be extended beyond 11.59pm on Thursday, labelling it a "day-by-day" proposition.
Health officials have identified 4200 primary close contacts across 290 exposure sites, with 77 per cent of those returning negative tests so far.
Victoria's Chief Health Officer Brett Sutton said the virus was rapidly moving and there had been substantial transmission in high-risk settings.
"We are neck and neck with this virus and it is an absolute beast," Professor Sutton said.
Three of the 11 new cases reported on Monday are linked to an outbreak at Arcare Maidstone, after a female healthcare worker at the northwest Melbourne facility earlier returned a positive result.
A 99-year-old resident is among them and she has since been moved to hospital with mild symptoms.
Federal Health Minister Greg Hunt said a second resident, an asymptomatic 95-year-old, was also being re-tested on medical advice.
The Altona woman's son and an unvaccinated colleague, who also worked at BlueCross Western Gardens in Sunshine from Wednesday to Friday, have tested positive as well.
Some 655 aged care residents died last year as a result of Victoria's devastating second wave of COVID-19, which sparked a 112-day lockdown.
The majority contracted the virus from staff, many of whom were working across multiple facilities.
Victoria's Health Minister Martin Foley said he would resist the temptation to criticise the federal government repealing a ban on private aged care staff working across multiple sites late last year.
But shadow federal health minister Mark Butler leapt at the opportunity.
"This latest outbreak in aged care is a direct result of Scott Morrison's gross negligence and dangerous complacency," he told reporters in Canberra.
Mr Hunt said 4.7 per cent of staff had worked across different aged care sites in Greater Melbourne, and the rules were reinstated last week when Melbourne's cluster emerged.
The Arcare Maidstone outbreak has also raised questions about the speediness of the vaccine rollout in private aged care homes.
Health Department Secretary Brendan Murphy in mid-April said he expected it to be completed in May, but only 85 per cent of private aged care residents have been vaccinated to date.
"The government needs to pick its act up or we're going to see more tragedies like what we're seeing right now across Victoria," Mr Butler said.
A third of Arcare Maidstone's 110 staff and 53 of 76 residents had received at least their first dose to Sunday, with second doses moved forward to Monday.
Unlike the first worker who had received her first dose, the other infected employee had not got her first COVID-19 jab as she was "on personal leave" when vaccinations took place.
The 99-year-old resident had also only received one dose of the Pfizer vaccine and was awaiting her second.
Four aged care facilities are now in lockdown including Arcare Maidstone, Blue Cross Western Gardens, and Royal Freemasons' Coppin and Footscray sites.