Melbourne will enter stage 3 lockdown on Thursday that could last for a week after a Covid cluster grew to 26 cases overnight.
Health officials and senior government ministers held crisis meetings on Wednesday night.
The number of cases linked to the Whittlesea cluster ballooned to 26, from 15 on Wednesday.
The state's Health Department confirmed the new infections on Thursday ahead of a government press conference to discuss the length of the lockdown - which will likely last seven days.
Schools are set to close, and Greater Melbourne residents will only be able to leave home for four reasons; exercise, going to work, caregiving and medical reasons, and buying food and essentials.
The lockdown, which could apply to Greater Melbourne or be statewide, will be officially finalised in a cabinet at a meeting at 9.30 am.
As of Thursday morning, there were 79 exposure sites listed on the Health Department's website, spread across more than 30 Melbourne suburbs and regional Bendigo, Cohuna, and Red Hill.
Contact tracers are continuing to hunt a 'missing link' in the infection chain, with the virus genomically linked to a man who caught it in Adelaide hotel quarantine.
An announcement by Acting Victorian Premier James Merlino is expected to come at around 9 am on Thursday, with Daniel Andrews still off work with a serious back injury.
The infectious drinker partied at three different bars in Prahran, in the city's southeast, on Saturday night - and ended up kicking on until 4 am on Sunday in one of the venues known for packed crowds and a pumping dancefloor.
The pub-crawler, unknowingly infected with a highly contagious double mutant Indian strain of the virus, began the evening at a cocktail bar on Chapel Street called Three Monkeys.
They arrived at about 9.15 pm and then wandered down the street to the famed Somewhere Bar - an establishment frequented by a sea of young, hip partygoers.
That Saturday the venue was packed as DJs blasted out tunes late into the night.
Three Monkeys also had a good turnout, raising fears the infected drinker may have sparked a super-spreader event.
'It wasn't crazy but it was solid,' owner Antony Harvey told The Age of the crowd that night.
The infected case stayed at Somewhere Bar for a slue of drinks before heading back to Three Monkeys at about 1 am.
The positive case then headed off towards Circus Bar - known for its raucous dance floor - on Commercial Road and kept the party going until 4 am
The MCG and Marvel Stadium are amongst the exposure sites and three inner-suburban nightclubs remain a 'concern'.
An infected person attended the Collingwood-Port Adelaide AFL game at the MCG on Sunday, while one was also at the North Melbourne-Essendon clash at Marvel Stadium later that same day.
A decision on crowd limits for this weekend's AFL matches in Melbourne has been delayed and the RISING festival has been cleared to go ahead, for now.
A number of restrictions were introduced for Greater Melbourne on Tuesday evening, including gathering limits of five people at homes and 30 in public and mandatory masks indoors.
Genomic sequencing shows the City of Whittlesea outbreak is linked to the case of a Wollert man, who contracted the virus in quarantine in South Australia earlier this month.
A report released on Wednesday in SA found it likely he caught the virus through aerosol transmission due to the 'timing of doors opening and closing' at an adjacent room occupied by an infected guest.
SA on Wednesday became the first state to close its border to residents of Greater Melbourne. NSW and Queensland are urging residents to reconsider travel to the city and parts of regional Victoria.
Victoria's last lockdown was for five days in February, after the virus escaped hotel quarantine and infected 13 people.
Patient zero for the latest Australian outbreak, who came to be known as the Wollert man, returned to South Australia from India on April 19, where he underwent 14 days in hotel quarantine at the Playford Hotel in Adelaide.
The man in his 30s tested negative to the virus three times during his stay and was discharged on May 4 before he travelled to his home in Wollert, in Melbourne's north.
But a few days later he developed symptoms and genomic testing would later reveal he contracted the virus from another hotel quarantine guest who was staying in an adjacent room.
SA Health handed down the findings of a report on Wednesday that suggested the virus likely spread when doors of adjacent rooms were momentarily opened at the same time.
His infection has since been gnomically linked to the new shock cluster gripping Melbourne.
But a definitive link between the Wollert man and the outbreak hasn't been established as contact tracers scramble to find a potential 'missing link'.
'The only missing link is the link to the Wollert case from the South Australian hotel quarantine,' Chief Health Officer Brett Sutton said.
While authorities have 'run down multiple generations of transmission', Professor Sutton is concerned about the amount of time that positive cases have been out in the community.
'There are potential exposures there that relate to some days ago that are yet to play out in new cases,' he said.
The Victorian government already introduced a number of restrictions to try and stop the spread including mask mandates indoors and limits on gatherings.
Other states around Australia have also taken steps to stop the virus from spreading, re-introducing tough border restrictions that require incoming travellers to self-isolate and Melburnians banned from South Australia altogether.
It's understood officials are considering a five-day circuit breaker, which could be extended if significant numbers of cases not directly linked to the current cluster emerge, The Herald Sun reported.
The fears Melbourne's Covid-19 outbreak could already have spread interstate come after health authorities confirmed a virus carrier attended sporting fixtures involving a New South Wales team.
NSW Health said it is urgently contacting people who attended a Central Murray Football and Netball League game between the Victorian club Cohuna and the New South Wales-based team Tooleybuc-Manangatang on Saturday afternoon.
Anyone who attended the footy and netball games, which were held in Victoria but with many fans who travelled from NSW, should immediately get tested and self-isolate.
Victorian health authorities said the positive Covid-19 case attended the Cohuna Recreation Reserve and watching multiple netball games alongside other spectators.
The infected person also visited the canteen and used the public toilets at the venue.
Health authorities are urging anyone who was at the event between 12 pm and 5 pm to get tested urgently and isolate until they receive a negative result.