Premier Daniel Andrews announced a five-day "short, sharp circuit-breaker" lockdown on Friday afternoon, with restrictions effective from 11.59pm.
People are only able to leave home to shop for food and essential items, provide or receive care, exercise and to work or study if they can't from home.
The rules are largely in line with stage four restrictions imposed last year, with a five-kilometre travel limit, compulsory face masks indoors and outdoors and no visitors allowed at homes.
The lockdown is in response to a COVID-19 outbreak at Melbourne Airport's Holiday Inn, which has grown to 13 cases.
Five new cases were recorded on Friday - an assistant manager at the quarantine hotel and four close contacts of people who earlier tested positive.
All five were self-isolating at the time they tested positive.
Genomic testing on earlier cases confirmed they have the more virulent UK strain of the virus.
The "working assumption" is all people linked to the cluster have the B117 strain, which Mr Andrews described as "hyper-infectious".
"It is the advice to me that we must assume that there are further cases in the community than we have positive results for," he told reporters on Friday.
The premier was confident the lockdown, similar to what was imposed in Adelaide, Brisbane and Perth after cases escaped quarantine, would be effective.
Authorities are working to track down 5000 people who passed through Melbourne Airport's Terminal 4 after a case worked at Brunetti cafe while infectious on February 9 between 4.45am and 1.15pm.
About 29 flights arrived and departed through the terminal during the eight-hour window.
Anyone who visited the terminal then needs to get a test and isolate for 14 days, including those who have since travelled interstate.
The outbreak can be traced back to a family of three who quarantined at the Holiday Inn and are believed to have been infected overseas.
One family member, who is now in intensive care, used a medical device called a nebuliser in their room, despite them banned outside of medi-hotels.
All international passenger flights have been cancelled from Saturday, excluding those already in transit on Friday.
Victorian health authorities issued new exposure sites late on Friday including the Coates Hire Werribee and Caltex Woolworths in Hoppers Crossing as well as the Craigieburn train line, the 513 Eltham to Glenroy bus route and the 901 Frankston to Melbourne Airport bus route.
Victorian Opposition leader Michael O'Brien criticised the Andrews government for locking down the state for a third time.
The Australian Small Business and Family Enterprise ombudsman Kate Carnell said the timing of the lockdown, coinciding with Valentine's Day and Lunar New Year, would hurt florists and restaurants.
In response to the outbreak, Tasmania, Northern Territory, Queensland, Western Australia and South Australia have closed their borders to Victorian travellers, while the NSW border remains open.
© AAP 2021
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