Victoria has recorded a further six local cases of COVID-19 in addition to the five detected overnight, as the virus seeps into aged care homes.
Acting Premier James Merlino said public health teams were "urgently investigating and interviewing" the cases.
The new cases take the cluster total to 51 infections.
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The infections detected overnight included a close contact of a previous case linked to the Star Car Wash and one is under investigation.
Three cases have also been linked to private residential aged care.
The number of exposure sites has exploded to more than 270 locations across the state.
"The challenge ahead of us is a very, very significant one," Mr Merlino said.
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"In the past 24 hours, we identified many more points of concern. In addition to the very worrying cases in private aged care, we are very concerned about the number of other high-risk exposure sites.
"We are seeing a small number of cases infecting a large number of contacts. There is no doubt, the situation is incredibly serious.
"The next few days remain critical. I want to be very clear with everyone, this outbreak may well get worse before it gets better."
More aged care staff positive
Three of today's COVID-19 cases are linked to private aged care, Health Minister Martin Foley confirmed.
The cases include a second staff member and a female resident in her 90s at Arcare Maidstone Aged Care, as well as the son of the first staffer who earlier tested positive.
The woman in her 90s had received one dose of the Pfizer vaccine.
However, the second Arcare worker did not receive the first dose on May 12 when other staff were receiving the jab, as she was on personal leave at the time.
The staff member also worked at the BlueCross Western Gardens Sunshine facility on May 26, 27 and 28.
The worker tested negative on their last day of work on Friday, but returned a positive result last night.
They are a close contact of the first infected staff member at Arcare Maidstone.
"We are closely monitoring all residents for any symptoms of COVID-19. At this stage, all are symptom-free," BlueCross CEO Robert Putamorsi said.
"Additional testing has been arranged for all residents and staff for today."
Arcare CEO Colin Singh said contact tracing was "well underway" at the Maidstone facility.
"We are closely monitoring our residents and team members for signs and symptoms and a second round of testing will be carried out tomorrow, Tuesday 1 June," Mr Singh said.
"We are doing everything possible to minimise any further exposure to residents and team members and have placed the Maidstone residence in lockdown."
The two aged care homes have been placed into a hard lockdown, with residents confined to their rooms as a testing blitz is carried out.
The resident in her 90s is displaying mild symptoms but will be transferred to hospital so she can be closely monitored.
The workers also have mild symptoms and are isolating at home.
Arcare confirmed 53 of the 76 residents at the Maidstone facility consented to get the jab and have received one dose of the Pfizer vaccine.
They will be receiving their second dose today. All staff will also be offered the jab.
The first Arcare worker's infection is a mystery case which remains of "extreme concern" to health authorities, who are scrambling to track down the source of the infection.
The first aged care worker was tested immediately after developing symptoms.
Testing commander Jeroen Weimar said it was still unclear how yesterday's mystery case emerged.
"We are still working on the linkages between the first aged care case," he said.
The Altona woman worked at the facility for two days while infectious, testing positive last Friday.
She had received one dose of the vaccine.
Other facilities on alert
An Arcare aged care worker's infection also prompted the Royal Freemasons Footscray and Coppin Centre facilities to go into lockdown after clinical staff worked at the same facility last week.
The Royal Freemasons this afternoon confirmed that person tested negative.
"The Coppin Centre staff member, a tier two contact, returned a negative test result this afternoon and the Department of Health advised that Coppin Centre can stand down and return to normal operations," a statement read.
"Both Royal Freemasons sites have now been cleared."
READ MORE: More than 270 exposure sites in Victoria
Chair of epidemiology at Deakin University Catherine Bennett told Today the case in the high-risk setting remained of concern.
"It is our worst fear – having someone not knowing they are infectious at work in aged care is exactly what you don't want to happen," she said.
Health authorities finding the missing link was among the factors necessary to prevent the lockdown from being extended, Professor Bennett said.
"We are certainly looking for the links for that case," she said.
"I think we are also looking to see increasingly if we are seeing cases – that they are going to be people who were quarantined when they had a negative test and are turning positive in quarantine.
"That's the best news because it means even if we, unfortunately, have more cases, we are in a situation where we are not adding to that public exposure list."
Chief Health Officer Professor Brett Sutton said it was "massively risky" to have staff working across multiple aged care facilities.
It comes as staff in federally-run private facilities have worked between several homes.
"It is a question for the Commonwealth, it is a Commonwealth workforce," Professor Sutton said.
"The Victorian public sector aged care, there has been both financial support and a policy setting that has effectively prohibited work across multiple aged care sites.
"That is entirely appropriate. It is massively risky to move across different settings."
Lockdown may be extended
Professor Sutton said extending Victoria's current snap lockdown was a "day-by-day process".
"It has been a rapidly moving virus and the transmission that has occurred in those high-risk settings has been very substantial," he said.
"So we have to take it as a day-by-day prospect.
"With more numbers today coming through and those really concerning settings, especially in aged care, we are neck and neck with this virus and it is an absolute beast."
New exposure sites released
More than 270 locations have been added to a rapidly growing exposure site list.
Authorities have identified 4200 primary close contacts and 77 per cent of people tested have already returned a negative test result.
Mr Weimar identified four new locations of concern, including Inday Filipino Asian Store and Thai Huy Butcher at Footscray Market, JMD Grocers & Sweets in Epping and Healthy India supermarket in Reservoir, which have all been listed as Tier One sites.
The health department has found 186 primary close contacts from a large birthday party at The Grand in Epping last weekend.
About 700 students and staff at Mount Ridley College in Craigieburn are also undergoing testing, 30 of whom are of particular interest.
The testing chief said of those 30, 23 have so far returned a negative test result.
A number of shopping centres and stores have been added to the Tier One list requiring 14 days of self-isolation and testing, including Aldi at Sanctuary Lakes Shopping Centre, Big W in Craigieburn and Coles at Point Cook Shopping Centre.
The state's largest shopping centre, Chadstone, along with Northland Shopping Centre on a Saturday afternoon and Sanctuary Lakes Shopping were added to the Tier Two list, urging anyone who attended the sites to get tested.
