Health authorities are scrambling to find the source, with a man in his 30s catching the virus after attending the sales at Chadstone Shopping Centre on December 26 and the Boxing Day Test at the MCG the following day.

Chadstone and the MCG, billed as the largest shopping centre and sports stadium in the country, have now been listed as possible transmission sites.

It presents a major challenge to Victoria's bolstered testing and contact tracing network, with the state's COVID response commander Jeroen Weimar describing it as a "needle in a haystack exercise".

Victoria has not had a mystery case since early November, as the state's deadly second wave wound down to usher in 61 days without community transmission.

The streak emboldened the state government to lift restrictions, signing off on a 30,000-strong crowd for each day of the Australia-India Test.

In light of the unknown case, patrons seated in zone five of the Great Southern Stand from 12.30pm to 3.30pm on day two have been asked to get tested and self-isolate until they receive a negative result.

They were expected to be notified with a text message on Wednesday using detailed ticketing information and the state government's QR code system.

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 have been issued the same advice.

Tracing efforts at the southeast Melbourne shopping precinct will be aided by tracking data from the man's mobile phone, to pinpoint his exact movements.

Mr Weimar estimates the case will impact 7000 to 8000 fans who were in zone five at the Test and "thousands" more searching for a bargain at the busy Chadstone Boxing Day sales.

The man has not visited NSW or any high-risk exposure sites in Victoria from the ongoing Black Rock cluster.

He was at a Christmas Eve function with someone 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 December 29.

The infected man developed symptoms on December 30 and has been at home and self-isolating since New Year's Eve.

He was tested late on Sunday before the result came back on Tuesday.

Since then, officials 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 infection leaves Victoria's Black Rock cluster, seeded from outbreaks in NSW, at 27 cases.

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.

Meanwhile, the Victorian government announced on Wednesday that its return-to-work plans would be paused for a week in response to the Black Rock outbreak.

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 February 8.

Commercial offices had previously been told they could ramp up half capacity from January 11.

Australian Associated Press