The Health Department on Tuesday confirmed a total of nine locally acquired cases, though six of these were already reported by Acting Premier James Merlino on Monday.

It brings the total number of active cases in the outbreak to 54.

Some 42,699 Victorians were tested in the 24 hours to Tuesday morning, while 20,484 were vaccinated.

There are now more than 300 exposure sites linked to the outbreak and 4000 close contacts self-isolating, of which about 80 per cent have returned negative tests.

Of greatest concern to authorities are three cases linked to an aged care facility in Melbourne’s northwest.

Two workers at Arcare Maidstone and a 99-year-old resident have tested positive for COVID-19, prompting the federal government to scramble to complete vaccinations in aged care facilities.

It was revealed on Monday the resident, who has been taken to hospital for treatment, received just one dose of the Pfizer vaccine.

One of the workers also received a dose but the other, who also worked shifts at BlueCross Western Gardens in Sunshine from Wednesday to Friday, has not.

Both facilities have been locked down as all staff and residents are tested.

Federal MP Bill Shorten, whose electorate of Maribyrnong takes in the Maidstone facility, said workers in both the public and private systems should have been vaccinated by now.

National cabinet will on Friday consider a proposal to make vaccination mandatory for all aged and disability care workers.

Meanwhile, contact tracers are yet to determine how the first aged care worker caught the virus, with a genomic sequencing report expected to be finalised as early as Tuesday.

When asked if the lockdown could be extended beyond 11:59pm on Thursday, Acting Premier James Merlino said the outbreak could get “worse before it gets better”.

Victoria’s Chief Health Officer Brett Sutton said authorities were “neck and neck” in the fight against the virus.

“It’s been a rapidly moving virus and the transmission that’s occurred in those high-risk settings has been very substantial, so we have to take it as a day-by-day prospect,” he said on Monday.

Professor Sutton would not rule out a return of the “ring of steel”, a series of road border checkpoints that separated Melbourne and regional Victoria during the state’s second wave of COVID-19 last year.

But the police union has come out against such a move, describing it as ineffective and inefficient.

“We think there’s a better way of doing it,” Police Association Secretary Wayne Gatt told ABC Radio Melbourne.

Two new cases of COVID-19 were also recorded in hotel quarantine on Tuesday.