At 11 a.m. local time, Acting Premier James Merlino declared the lockdown (1pm NZT).
Other than an intimate spouse, Victorians would not be allowed to have visitors in their homes. There will be no public meetings, but takeout will be available from restaurants, pubs, and cafes.
Unless an exemption is granted, masks must be worn anywhere during the lockdown, including indoors and outdoors.
There are currently 34 ongoing cases, with 40,411 tests processed yesterday, the largest number ever in Victoria.
"We've had 12 associated new cases in the last day," Merlino said, "increasing the total number of cases linked to the hotel quarantine breach in South Australia to 26."
"We've seen further indications in the last day that we're dealing with a highly infectious strain of the virus, a variety of concern, that's spreading quicker than we've ever seen."
"We've identified over 10,000 primary and secondary contacts who will need to be quarantined, tested, and isolated since first thing this morning, and that figure will continue to rise and alter."
"The time between contracting a virus and transmitting it is shorter than it has ever been. The time between the development of symptoms in the primary and secondary instances in some of these cases is little over a day.
"To put things in context, the average transmission time is roughly five to six days."
Authorities in South Australia imposed a hard border with residents of Greater Melbourne on Wednesday, after it was established that the man at the center of the epidemic most likely caught it in an Adelaide hotel quarantine.
The last time Victoria's restrictions were strengthened was in February when a 13-case epidemic triggered a five-day lockdown.
While the fact that all of the cases in this epidemic have been related to one another is a good indication, authorities are concerned about a growing list of high-risk exposure sites linked to the new infections.