Victorian coronavirus outbreak: How did we get here?

The coronavirus outbreak, linked to a hotel quarantine breach, began in Melbourne's northern suburbs but in recent days the cluster has exploded, with the number of cases doubling in just 24 hours.

A list of more than 150 exposure sites details bars, stadiums, gyms and retail stores across the north and south-east of the state, as well as some regional areas around Bendigo.

The highly-infectious strain has authorities concerned, plunging Victoria into a seven-day "circuit breaker" lockdown to allow contact tracers to get on top of thousands of contacts identified from the cluster.

So what exactly is happening, and how did we get here?

It all began with the case of a man who was infected in hotel quarantine in South Australia earlier this month.

The man from Wollert in Melbourne's outer north, tested positive after returning from India via the Maldives and Singapore on April 19 and completing hotel quarantine in Adelaide.

He was staying in a room next to a person who tested positive before being moved to a medi-hotel.

A report released on Wednesday 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.

Genomic sequencing has linked the new cases to that Wollert man, however there is still a missing link.

Victorian Chief Health Officer Brett Sutton said detailed interviews of each case had yet to yield results.

"We will continue to try and find links," he said.

"If we can then we will know if there are other people exposed who need to get tested, but detailed interview and re-interview has not identified the exact link."

There were 11 new cases reported on Thursday, doubling the size of the outbreak to 26.

One person is in intensive care on a ventilator.

The virus is moving quickly, there are already more than 150 exposure sites identified and many of those are high-risk areas like bars, gyms and football stadiums.

Professor Sutton was reassured all the cases are linked.

He said one exposure site was a finance firm in Port Melbourne, while all the other cases are from related households or casual links.

However, more than 10,000 primary and secondary contacts of the reported cases have been identified.

Professor Sutton said that number hadn't been seen before.

"Part of the reason for the big numbers we've seen is there were generations of transmission that occurred before the first case was identified to us," he said.

The variant of the virus running rampant through Victoria is a highly infectious Indian strain called the B161 variant.

Victoria's acting Premier James Merlino said it was running through the community "faster than we have ever recorded".

"Our public health experts' primary concern is how fast this variant is moving. We've seen overseas how difficult that movement can be to control. Here in Victoria, we're seeing not only how quick it is, but how contagious it is as well."

He said the time between catching the virus and showing symptoms is averaging just over a day.

"To put that in some perspective, the usual transmission is about five to six days. In some of these cases, within a day it's being transmitted."

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Professor Sutton said the B161 strain was "at least as infectious as the UK variant and possible more infectious."

"That was challenging enough. We do not want to have to deal with variance of concern like this again."

Part of the problem is the type of venues infected people have attended.

Many of the exposure sites - including clubs, football games and gyms - are crowed places, where people dance, sing and chant, causing a higher-risk.

Western Australia: Anyone who has been in Victoria since May 16 will be banned from the state unless they have an exemption. Those granted exemption must quarantine.

South Australia: The border is shut to travellers from greater Melbourne. Essential workers and South Australian residents can go home but must quarantine for 14 days.

Tasmania: Another closed border for anyone who has been in Victoria in the past 14 days. Travellers can apply for an exemption but quarantine is required. Tasmanian residents can apply for the exemption and quarantine at home.

Queensland: All Victorian travellers must complete a border pass. Arrivals who have been in the City of Whittlesea since May 11 need an exemption. Returning residents, or anyone with an exemption must go into hotel quarantine.

Northern Territory: Anyone arriving from greater Melbourne and Bendigo from midnight Thursday must quarantine at the Howard Springs or Alice Springs facilities.

NSW: Anyone arriving from Victoria after 4pm Thursday will be subject to the Victorian lockdown rules and stay at home unless leaving for essential reasons. They must also complete a declaration form.

ACT: Non-residents from Victoria are banned without an exemption. Returning residents must follow Victoria's lockdown rules and stay at home unless leaving for essential reasons. Travellers from the City of Whittlesea must isolate until a negative test result is received.