Victorian authorities say people are becoming infected with Covid after just "brushing past" strangers with the virus.
Testing Commander Jeroen Weimar said at least four of the state's 54 locally transmitted cases have come from "fleeting" contact between Victorians.
"What we're seeing now is people are brushing past each other in a small shop, they are going to a display home, they are looking at photos in a Telstra shop," he said.
"This is relatively speaking, relatively fleeting. They do not know each other's names, and that is very different from what we have seen before."
He said the ease with which the virus is spreading may be a feature of the Indian variant.
"We are used with previous variants, we are more used to transmission occurring in the home, in the workplace, where people know each other already, not all of those big social settings," he said.
"These are quite different."
"We have seen transmission in these places with very fleeting contact. We have transmission in places like the Telstra store in South Melbourne, JMD Grocers, the display home we talked about a few days ago, I'd add Craigieburn Central shopping centre."
Five-day vaccination 'blitz'
Victorian authorities today announced a five-day vaccination "blitz" as health officials work to combat the latest cluster emerging in Melbourne.
Fifty-three cases have been linked, with a further three local infections announced today.
Minister for Disability, Ageing and Carers Luke Donnellan announced the state government would be prioritising workers in vulnerable settings over the next five days.
"This means from Wednesday 2 June to Sunday 6 June, workers and private sector HM facilities and the residential disability sector, all managed by the Commonwealth, will be given priority access to walk-in vaccination hubs around Victoria between 9am and 4pm," he said, addressing the recent criticism over the lack of vaccinations available to aged care workers.
"Aged Care and disability ability workers are currently eligible to be vaccinated at state hubs, but this is about stimulating that demand and getting more workers to get greater coverage at the private aged care sector and the disability sector," he continued.
"We have concerns there isn't enough coverage, that is why we are stepping in."
Melbourne's coronavirus cluster has now 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.
Contact tracers are still trying to figure out how a staff member at Arcare, a woman in her 50s, picked up the virus.
Three new community cases today
Of the three new locally acquired Covid cases reported today, two were primary close contacts of previous cases and were already quarantining.
The final case is under investigation.
Victorian Health Minister Martin Foley said authorities were still assessing the state's coronavirus outbreak on a "day-by-day basis".
"There continues to be significant concerns around ongoing transmission and particularly our northern suburbs and many large exposure sites that have been added to the exposure site list over the last 36 hours," he said.
"Our public health team are currently weighing whether there is still coronavirus circulating in these communities and we will still consider our response to the current outbreak on a day-by-day basis."
There are now more than 320 exposure sites across Victoria. A "significant number" of exposure sites were added to the list yesterday, including multiple bus routes, a DFO shopping outlet and a Big W store.
Despite the small number of new cases reported today, Victoria's mystery cases and growing list of exposure sites have heightened fears the state's "circuit breaker" seven-day lockdown could be extended.
The lockdown is due to end at 11.59pm on Thursday local time.
Opposition Leader Michael O'Brien weighed in on the state's outbreak yesterday, saying it seemed clear the lockdown would last "a lot longer" than seven days.
"It's a very concerning day for Victorians. The pretty clear message is that we can't look forward to restrictions being eased on Friday," he said, according to the Herald Sun.
The government has continually refused to be drawn on whether the lockdown will end on Thursday night as planned, with Acting Premier James Merlino saying it wasn't just the case numbers that would determine if restrictions are extended.
"It's the type of cases. It's where it's occurring. It's [whether] we know where they're linked, if are they high-risk sites," he said yesterday.
"All of those things are taken into account by the public health team in terms of when they're confident to provide advice to government that we can then go towards some easings of restrictions."
Merlino said the outbreak "may well get worse before it gets better".
Chief health officer Professor Brett Sutton said Victoria was "neck and neck with this virus", describing it as an "absolute beast".