Victoria is now grappling with 11 cases of coronavirus in the past 24 hours, with hopes the state's lockdown could still end at midnight on Thursday seeming less and less likely.
Acting Victorian Premier James Merlino said the state had recorded another six cases after the midnight cut-off and health authorities were "urgently" investigating them.
The state is now dealing with 51 community transmission cases linked to the Melbourne outbreak, with another nine in hotel quarantine.
Merlino said the situation in the state was "incredibly serious" and it was likely the outbreak "may well get worse before it gets better".
The outbreak has spread to four aged care homes — with Arcare Aged Care Home, in the west Melbourne suburb of Maidstone, now having three cases linked to it.
The home was put in lockdown over the weekend after a worker, a woman aged in her 50s, tested positive to coronavirus late Saturday night.
Arcare today confirmed a second team member and a resident had since tested positive to coronavirus.
The resident had received one dose of the Pfizer vaccine, but the second team member had not received any vaccinations.
The aged care resident is displaying mild symptoms and has been taken to hospital as a precaution.
"Arcare is well prepared for this outbreak and our infection control practices position us well to manage this effectively," the aged care provider said.
Another of today's cases is a worker at the BlueCross Western Gardens aged care home in Sunshine, the suburb next to Maidstone in Melbourne.
The staff member tested positive late last night and is a close contact of the care worker who tested positive at Arcare.
In a letter to staff, BlueCross' CEO said 76 per cent of residents had received their first coronavirus vaccine.
And the Coppin Centre Royal Freemasons aged care home, along with the Royal Freemasons Footscray aged care home, are now both in lockdown after its staff worked in a home where a positive coronavirus case has been identified.
Kerri Rivett, the chief executive of the aged care provider, said the homes had brought in "full outbreak procedures".
"We have just been notified that clinical staff members at our Coppin Centre and Footscray homes have, last week, worked at another aged care provider where a staff member has now tested positive for Covid-19," she said.
"The potential exposure occurred during work at the other provider and all staff were wearing a mask. Within our homes, staff have also been wearing masks at all times."
Fears of extended lockdown over aged care cases
Earlier today, health experts admitted the mystery case found in the Arcare Aged Care Home in Maidstone was extremely "concerning" as contact tracers rush to figure out how the employee caught coronavirus.
A massive health response, including a testing blitz of all staff and resident, was launched on Sunday morning after it was revealed the woman had inadvertently worked for two days while infectious.
Leading infectious diseases expert Professor Sharon Lewin said the mystery case in the aged care home was "concerning" but Melburnians should take solace in the fact that she had received the first dose of the Covid-19 vaccine.
"The mystery case is certainly concerning. [Firstly], it's a mystery case, so we need to know where this person acquired Covid. And second of all — they work in an aged care home which is the most vulnerable setting for Covid," Lewin told ABC News Breakfast.
Lewin said it was encouraging that contact tracers continued to link Victoria's new cases — but the aged care mystery case could mean lockdown is extended.
"Thousands and thousands of tests have been done, which is fantastic. And up until this one mystery case, they have all been linked which is also very reassuring," she said. "I'm very hopeful that we won't need to extend the lockdown for much longer should this sort of pattern even continue over coming days."
Arcare Aged Care Home in Maidstone was also hit by a coronavirus outbreak last year and had 14 infections linked to the home — including seven staff.
"While we hoped that this would not happen again, we are well prepared, and our infection control practices put us in a good place to manage this outbreak effectively," Arcare chief executive Colin Singh said in a statement.
Health staff were seen at the aged care home doing a deep clean over the weekend and residents were kept in their rooms.
Victoria's coronavirus testing commander Jeroen Weimar said the mystery aged care case was the state's top priority.
"It is our most vulnerable and sensitive setting that we have and that is why we have put such an important response into this," he told reporters yesterday.
"I am concerned that we don't have, at this point of time, an acquisition source. "This is the biggest priority for us to understand."
Victoria yesterday recorded five new local cases of Covid-19 on the third day of the state's lockdown.
Under current plans, the last day of lockdown will be Thursday, with the possibility of some restrictions extending beyond that.
New Zealand paused quarantine-free travel with Victoria amid the current outbreak, which will last until Friday, June 4, given there are no further changes.
Australian Prime Minister Scott Morrison arrived in New Zealand yesterday, and it is expected he would speak with Prime Minister Jacinda Ardern about extending the transtasman bubble beyond the two countries.
Morrison told reporters yesterday that "the idea of a bubble that goes beyond New Zealand and Australia is a real possibility".
"Fiji is going through a difficult time at the moment and we're supporting them to come through that, but whether it's Vanuatu, or the Solomon Islands or Fiji or Tonga, or any of these places, and even supporting up in Timor Leste.
"Now, these are real challenges to overcome, but when it comes to people coming to work in Australia, the seasonal workers programme ... or the broader partnership that we have across the Pacific for other purposes - this is why we get together every year."