Health Minister Martin Foley confirmed a further six cases were detected past the midnight deadline, on top of the five already reported on Monday.

There still has not been a case in regional Victoria amid the latest outbreak.

Three of those cases are linked to an outbreak at the Arcare Maidstone after a worker at the facility returned a positive result on Sunday.

The first is the son of the Altona woman and the second is an unvaccinated colleague who also worked at Blue Cross Western Gardens facility in Sunshine on Wednesday, Thursday and Friday.

See the full list of exposure sites here.

An Arcare Maidstone resident, a woman in her 90s, has also tested positive and has 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 told reporters on Monday.

"It's a very great concern to all Victorians and indeed to the Commonwealth government."

There were no announcements made today about whether the current lockdown will extend beyond 11.59pm on Thursday - when it was due to end.

On the fourth day of the state's seven-day lockdown, Acting Premier James Merlino warned the situation was worsening as active cases hit 54.

"There is no doubt, the situation is incredibly serious," he said.

"The next few days remain critical. I want to be very clear with everyone, this outbreak may well get worse before it gets better."

Arcare chief executive Colin Singh said the resident had received one dose of the Pfizer vaccine and was awaiting a second dose.

Blue Cross Western Gardens has now joined the Arcare Maidstone in locking down residents.

Contact tracers are yet to uncover how the first worker caught the virus, making her a "mystery case".

Only a third of Arcare Maidstone's 110 staff and 53 of 76 residents have been vaccinated so far, with the federal government bringing forward scheduled vaccinations to Monday.

Federal MP Bill Shorten, whose electorate of Maribyrnong takes in the facility, said most residents and staff were yet to receive their second dose of the vaccine.

"The people who are sick had actually had a vaccination. You need two. One is not enough," he told ABC News Breakfast on Monday.

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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 hindsight of almost 2000 Victorian aged care residents contracting COVID-19, 655 resident deaths and more than 1600 aged care workers infected was not enough to motivate the Morrison government into urgent action," she told ABC Radio.

Meanwhile, Royal Freemasons have locked down its Coppin centre and Footscray facilities after it was notified a staff member had worked at Arcare Maidstone.

More than 600 people living in aged care homes died last year as a result of Victoria's second wave of COVID-19.

The majority contracted the virus from staff, many of whom were working across multiple facilities.

There are now 290 locations on the state's list of exposure sites, including a soft drink factory in Thomastown, a number of bus routes in Melbourne's north and various stores at Chadstone shopping centre, as well as Arcare Maidstone.

Craigieburn's Willmott Park Primary School was closed on Monday with nearby Mount Ridley College listed as an exposure site after a pupil tested positive for COVID-19 on Saturday.

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