Victoria's contact tracers are working frantically to track the rapidly spreading Covid strain sweeping across Melbourne.
With a further 11 local cases confirmed on Thursday, taking the state's latest outbreak to 26 cases, Victoria will be locked down for seven days beginning at midnight.
The Covid Indian variant is spreading so fast contact tracers on Thursday morning had identified in excess of 10,000 primary and secondary contacts who have crossed paths with one of the infected people.
Acting Premier James Merlino said the thousands of Victorians identified as contacts would need to quarantine, or get tested and isolate until their results.
“That number will continue to grow and change,” he warned.
“The time between catching the virus and passing it on is tighter than ever,” he said.
Mr Merlino said this variant was being transmitted within the same day someone catches it.
"The usual transmission is about five to six days," he said.
"The number of cases has doubled in 24 hours."
Unless something drastic happens, this will become increasingly uncontrollable.Acting Victorian Premier James Merlino
Victoria's Chief Health Officer Professor Brett Sutton said the speed of infection of the Indian variant was particularly concerning.
"It really is rapid, and that's led to the exponential increase (of contacts)," he said.
Prof Sutton strongly defended Victoria's contact-tracing efforts as the outbreak spreads, saying it was an "absurd proposition" the system was not working properly.
More than 150 exposure sites in Victoria
With more than 150 exposure sites right around the state, Victoria “needs to act now”, Mr Merlino added as he announced the state's 7-day lockdown.
The MCG and Marvel Stadium are among the list of exposure sites.
There were infected people at AFL matches at the weekend, while another visited three bars in Prahran and South Yarra, which is of concern to authorities.
Mr Merlino noted the current outbreak originated from South Australian hotel quarantine.
Genomic sequencing shows the outbreak is linked to the case of a Wollert man, with an official South Australian report on Wednesday suggesting he caught the virus from aerosol transmission in an Adelaide quarantine hotel.
But a definitive link between the Wollert man and the Whittlesea outbreak has not been established.
Western Australia, South Australia and Tasmania have temporarily closed their borders with Victoria.
All of Victoria in 7-day snap lockdown
People will only be able to leave home to shop for food and essential items, provide or receive care, exercise, work or study if they are unable to from home, and get vaccinated.
The 5km travel limit will also be reimposed for exercise and shopping, as will compulsory use of masks indoors and outdoors.
All non-essential retail will close but essential stores including supermarkets, bottle shops and pharmacies will remain open.
Cafes and restaurants will only be able to offer take-away.
Childcare and kinder will stay open, but schools will close.
Professional sports events such as the weekend's AFL games in Melbourne will continue, but without crowds.
Covid vaccine program expands
Mr Merlino also announced the state's vaccine program will expand, with people over 40 now eligible for the Pfizer jab.
In his media conference, Mr Merlino highlighted the slow rollout of the Commonwealth vaccine program and continued problems with hotel quarantine.
"We are not through this pandemic until we have had the successful rollout of the federal government's vaccine programs and we have safe alternatives to hotel quarantine," he said.
While the lockdown could be shorter than seven days, Chief Health Officer Brett Sutton said he is doubtful.
"We have to ready ourselves for seven days of this lockdown," Professor Sutton said.
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