Victorian Chief Health Officer Brett Sutton said reinstating a "ring of steel" between metropolitan Melbourne and regional Victoria was "not out of the question", as the Melbourne outbreak worsens.
Victoria has recorded 11 new locally acquired COVID-19 cases today, with a possible extension to the state's snap lockdown to be considered "day by day" as Melbourne's outbreak grows.
While five new community cases were initially reported this morning, Acting Premier James Merlino said a further six locally acquired cases came through after the late-night cut-off.
Mr Merlino said these additional cases were being urgently investigated and interviewed, with further details to be released soon.
He warned the outbreak "may well get worse before it gets better".
"There is no doubt, the situation is incredibly serious," he said.
"The next few days remain critical."
Asked whether the state's seven-day lockdown would be extended, Professor Sutton said it was a "day by day" prospect.
"As the Acting Premier said, it is concerning numbers and concerning settings," Professor Sutton said.
The circuit-breaker lockdown is currently scheduled to finish at 11.59 pm Thursday.
With zero positive cases in regional Victoria, Professor Sutton was asked if the regions should have a different set of rules from Friday if the lockdown was extended.
"We will see when we get there, we don't know what will emerge tomorrow," he said.
"There are a number of primary close contacts in regional Victoria, a number who have tested negative but we don't know what might be around the corner."
He said the reintroduction of a ring of steel between metropolitan and regional Victoria would be determined as the situation was reviewed day by day.
Health Minister Martin Foley confirmed three of today's new cases were related to emerging outbreaks in private residential aged care.
One is the son of a staff member at aged care Arcare Maidstone, an Altona woman in her 50s who tested positive for COVID-19 on Sunday.
A second worker at the aged care facility who worked alongside this initial case has also tested positive - Mr Foley confirmed she was not vaccinated.
This employee also worked at another aged care facility, the Blue Cross ACG Sunshine facility, from May 26 to 28, triggering "a strong public health response".
The third case is a resident at Arcare Maidstone, a woman in her 90s who has been transferred to a separate hospital.
Arcare Maidstone is currently in lockdown, with residents placed in self-isolation.
The Royal Freemasons' Footscray and Coppin Centre aged care homes also locked down this morning after it was discovered two staff members worked at Arcare Maidstone while the facility's first infectious employee was on shift.
It's believed this first case may have been infectious during shifts on Wednesday and Thursday, however contact tracers are yet to discover how she caught the virus, making her a concerning mystery case.
These new cases have sparked growing concern about aged care staff members working across multiple sites.
Mr Merlino said while movement across sites wasn't permitted in the public system, private aged care was the responsibility of the Federal Government.
There are now more than 4200 primary close contacts and 270 exposure sites in Victoria linked to the growing outbreak.
Today the Victorian Health Department reported more than 43,874 tests were conducted in the 24 hours to midnight, with more than 16,700 COVID-19 vaccine doses administered in the same period.