Victoria will enter a "short, sharp circuit-breaker" lockdown for five days to respond to fears the highly infectious UK strain of coronavirus has spread throughout the community, says Premier Daniel Andrews.
From midnight tonight (local time), all of Victoria will return to stage 4 lockdown restrictions, meaning there are only four main reasons to leave the home: shopping for essential supplies, care and caregiving, exercise and essential work.
Exercise and shopping will be limited to within 5km of the home.
Face masks will need to be worn indoors and outdoors if you are outside your own home, and no visitors will be allowed in homes.
Under the rules, schools and tertiary education will be closed, public gatherings are banned, people must work from home when they can, and weddings will be banned except for on compassionate grounds.
Places of worship and religious gatherings and ceremonies will not be permitted, and funerals will be capped at 10 people.
Andrews confirmed people would be fined for breaches of the rules.
The outbreak linked to the Holiday Inn quarantine hotel at Melbourne Airport has grown to 13 cases, with five new cases confirmed on Thursday.
The Holiday Inn outbreak is thought to have started when an infected returned traveller used a nebuliser - an aerosol-creating medical device banned in hotel quarantine - which caused the virus to spread throughout the hotel floor they were on.
The first cases have all been confirmed to have the more virulent UK strain of the virus and the "working assumption" is all people linked to the cluster have the same B117 strain.
All of the locally acquired cases were linked to the outbreak, with workers from the hotel and their close contacts confirmed to have contracted the virus.
Andrews said the "hyper-infectious" UK strain was presenting in a manner that "is a very significant concern to us".
"I am sad to have to report, it is the advice to me that we must assume that there are further cases in the community than we have positive results for," Andrews said.
"And that it is moving at a velocity that has not been seen anywhere in our country over the course of these last 12 months."
Andrews said by the time positive cases had been detected by contact tracers, "they have already infected their close contacts, their family, people they live with, people they spend time with that makes it incredibly difficult to do contact tracing because there is no gap between the first case, their close contacts and people they spent time with".
The Premier said the five-day lockdown, similar to snap restrictions imposed in Brisbane and Perth, was due to end at 11:59pm on Wednesday, but if cases dropped off rapidly the lockdown could end sooner.