Victorian health officials have called for an entire zone of spectators in the MCG's Great Southern Stand come forward for testing and quarantine amid fears that a mystery case of Covid-19 may have acquired the virus on the second day of the Boxing Day Test.

The case, a man in his 30s who also shopped at Chadstone Shopping Centre in Melbourne's eastern suburbs on Boxing Day, was seated in Zone 5 of the MCG under its covid-safe protocols - likely entering the ground through gates five or six - between 12.30pm and 3.30pm on December 27, before developing Covid-19 symptoms on January 5.

As a result of this case, the New South Wales government has made the wearing of masks mandatory at all times for the crowd at the SCG Test except when eating and drinking although the game will continue with a crowd at 25% capacity. Anyone who attended the MCG at the date and time of the alert is banned from attending the Sydney Test. There had already been a ban on people attending from certain suburbs of Sydney based on the outbreaks in the city.

"Based on the information available from the case, the man in his 30s, was not infectious while at the sites, but there is potential he acquired the virus while there," the Victorian Department of Health and Human Services said in a statement. "The man developed symptoms on 30 December and the department was notified of his positive test on 5 January. Given he has not visited any high-risk Victorian exposure sites or travelled to New South Wales, a number of acquisition sources are being investigated.

"The man was present at the MCG on 27 December, the second day of the Boxing Day test and is currently isolating. He was not infectious when he attended and is not linked to any other case or to the Black Rock Restaurant Outbreak. The MCG is being investigated as a potential source for the infection.

Health authorities asked for anyone who had been present in the affected zone of the MCG to get tested. "We're encouraging anyone who was in The Great Southern Stand, zone 5 of the MCG between 12.30pm and 3.30pm on 27 December, to get tested and isolate until they receive a negative result.

"A Covid-safe event plan was in place for the match and the ticketing allocation system in place has enabled effective contact tracing. The venue also utilised the Victorian Government's QR code system in an additional measure to support contact tracing.

"The department is working with Melbourne Cricket Club to contact ticket holders in the relevant area directly and to ensure that all relevant public health actions have been undertaken."

Announcing the updated masks requirement for the SCG, NSW health minister Brad Hazzard said: "From the moment effectively you get into the transport to get there, and get into the queue at the front door of the SCG, and go to your seat, and sit in the your seat, you must wear a mask.

"The only exception to that [is] if you're eating or drinking. Obviously, Dr Chant and the public health team want people to enjoy a day at the cricket, but it's a Covid day at the cricket and that means mask on, not mask off, unless you're eating or drinking, in which case you can have your mask off."

Australia captain Tim Paine was asked whether he believed continuing the Sydney Test with even a reduced crowd was safe as cases of community transmission continued to be recorded in New South Wales, but he deferred to those with more knowledge of the situation.

"It's not really an area I can control or worry too much about. People who are in control of making these decisions [are] at government level," Paine said. "There's health experts, high level people at Cricket Australia and they're all working as hard as they possibly can to do the best thing.

"Obviously we want to get people in the gates, give people a chance to come in and watch international cricket and if they're saying the safe number is 10,000 then I certainly can't question that. Don't have a medical background so we trust that the people who are in those positions are making the right decisions by us and the communities. We'll go with whatever we're told at this stage."