Victoria has recorded five new locally-acquired cases of COVID-19, four of of them close contacts of a food delivery driver who travelled across north and southeast Melbourne while potentially infectious.
But the state's health minister believes authorities are "getting close on the heels" of the outbreak as more exposure sites are revealed.
The new cases were diagnosed on Friday - the first of seven days scheduled for Victoria's fourth lockdown - from more than 56,000 tests.
The testing tally broke the state's daily record, while a record more-than-21,600 vaccinations were administered.
Most infections are linked to Melbourne's City of Whittlesea cluster, with cases testing positive for the B1617 strain first identified in India.
Health Minister Martin Foley told reporters four of the five local cases on Friday were connected to a delivery driver earlier confirmed COVID-positive who worked for numerous days while infectious.
"They either live in the same household or there was a meeting of households, quite appropriately, where transmission occurred ... (and) one of those is a student at Mount Ridley College (in Craigieburn)," he said.
The final case was a person connected to the Port of Melbourne outbreak linked to Stratton Financial. A worker at the financial firm visited a Mickelham display home and infected a worker there, Mr Foley said.
At least 14 Stratton employees are now COVID-positive.
While refusing to speculate on an extended lockdown, Mr Foley said he was confident authorities were bringing the issue under control.
He said close contacts of cases who visited Highpoint shopping centre in northwest Melbourne and two AFL games were returning negative results.
"We do not know what is in store in the next 24 hours but are making exceptional progress in terms of contact tracing and we are, I think, getting close on the heels of this outbreak," Mr Foley said.
Masks are mandatory in Victoria and residents can't leave home except to shop for essentials, provide or receive care, exercise, work or study, or to get vaccinated.
But scores of Melburnians queuing for the COVID-19 jab are facing long waits after technical issues crippled booking systems.
Some walk-ins reported being turned away from vaccine centres and others said they'd been warned of five-hour delays.
It comes after the state's coronavirus hotline was flooded with more than 77,000 calls in 15 minutes when it was announced on Thursday eligibility for the Pfizer vaccine would be expanded to people aged 40-49.
The Victoria Department of Health's Jeroen Weimar said the state was unable to administer more than roughly 20,000 jabs a day and booking system issues would be addressed by new software.
The strict measures have sparked anti-restriction rallies.
There were multiple injuries and 14 arrests when about 150 people gathered in the CBD to protest against the lockdown.
Two police officers sustained minor injuries during the arrest of six protesters in Victoria Street, North Melbourne.
One of the alleged offenders was also injured during the incident.
A total of 55 people were issued penalty notices in relation to breaching directions.
"Those who choose to blatantly disregard the CHOs directions and put the health and safety of all Victorians in jeopardy can expect to be held accountable and fined," Victoria Police said in a statement.