Victoria is embarking on a "needle in the haystack" search for the source of the mystery coronavirus case who attended the Boxing Day sales and day two of the MCG Test.
The COVID-positive man in his 30s is understood not to have been infectious during his visits to the Chadstone Shopping Centre on 26 December and the MCG on 27 December for the Australia-India Boxing Day Test.
Health authorities instead believe he may have caught the virus while in attendance, with Chadstone and the MCG listed as potential acquisition sites.
It is Victoria's first mystery case since early November and is expected to put more strain on testing sites, with thousands of people being urged to submit samples.
"We're trying to identify the needle in a haystack exercise of who might be positive," Victoria's testing boss Jeroen Weimar told reporters on Wednesday.
Patrons seated in zone five of the Great Southern Stand from 12.30pm to 3.30pm that day have been asked to get tested and self-isolate until they receive a negative result.
They are expected to receive a text message on Wednesday, based on detailed ticketing information.
Chadstone shoppers at Culture Kings, Huffer, JD Sports, Jay Jays, H&M, Uniqlo, Myer, Superdry, Footlocker and Dumplings Plus stores from 6am to 2pm on Boxing Day are also urged to get tested.
The man's mobile phone movements around the precinct will be tracked on Wednesday to pinpoint exact times for each shop.
Mr Weimar estimates the case will affect the 7000-8000 fans who were in zone five at the Test and "thousands" at the busy Chadstone Boxing Day sales.
Where the man acquired his infection is a mystery, given he has not visited NSW or any high-risk exposure sites in Victoria.
He was at a Christmas Eve function with someone present from NSW, although Deputy Chief Health Officer Allen Cheng said that person had not reported feeling unwell.
Nor is the case believed to be linked to a COVID-positive Boost Juice staffer at Chadstone who last worked on 29 December.
The infected man developed symptoms on 30 December and contact tracing has revealed he had stayed home and self-isolated since New Year's Eve.
He was tested late on Sunday before the result came back on Tuesday.
Since then, health authorities have been working to identify close contacts including an unwell housemate whose test result remains pending.
Professor Cheng said health authorities were most concerned about distinguishing "upstream contacts" in the five-to-six days before the onset of symptoms.
"We're not worried about these people becoming unwell, so much as they might have been unwell then and passed the infection to him," he said.
The unlinked local case leaves Victoria's Black Rock cluster total at 27.
A further two overseas acquired infections were reported on Wednesday, bringing the state's active case total to 41.
Testing rose for a third straight day, with 37,509 swabs processed on Tuesday.
The Victorian government also announced on Wednesday that the return-to-work plan will be paused for a week.
The outbreak linked to a Black Rock restaurant in bayside Melbourne, seeded from outbreaks in NSW, prompted the call for a review on Tuesday night.
The public service was due to move into the next phase of return to work from Monday - increasing office capacity to 25 per cent, ahead of a jump to 50 per cent on 8 February.
Commercial offices were to be allowed up to 50 per cent capacity from 11 January.
People in Australia must stay at least 1.5 metres away from others. Check your jurisdiction's restrictions on gathering limits.
If you are experiencing cold or flu symptoms, stay home and arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080. News and information is available in 63 languages at sbs.com.au/coronavirus.
Please check the relevant guidelines for your state or territory: NSW, Victoria, Queensland, Western Australia, South Australia, Northern Territory, ACT, Tasmania.
