Thousands of cricket fans who attended Australia's showpiece Boxing Day Test against India have been told to get tested and isolate Wednesday, after a spectator at the match tested positive for Covid-19.

Health authorities said the man in his thirties was not infectious while at the famed Melbourne Cricket Ground on the second day of play "but there is potential he acquired the virus while there" or at a nearby shopping centre. 

"The MCG is being investigated as a potential source for the infection," Victoria's Department of Health said.

A race is now on to trace and test an estimated 7,000-8,000 fans, and to find out if one of the highest-attended sporting events since the pandemic began was a super-spreading event.

"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," health authorities said. 

The Boxing Day Test is a centrepiece of Australia's sporting calendar and just under 30,000 people attended this year, well short of capacity.  

Several retail stores in the Chadstone Shopping Centre were also named as possible sites for the source of the infection.

Despite having been largely free of Covid-19, Australia is currently battling to bring a cluster of cases under control and hoping aggressive testing and contact tracing can help avoid full-scale lockdowns. 

The news comes on the eve of the third Test between Australia and India, which has caused some controversy by being played in Sydney, the epicentre of a recent outbreak.

The number of spectators has been restricted to 25 percent of seating capacity in order to make sure it does not become a mass spreading event. 

Authorities in New South Wales said the revelation from Melbourne was "obviously influencing our thinking" about how the Sydney Test would proceed, without elaborating.

Those attending will now have to wear a mask while seated, in addition to an existing requirement to mask-up while getting to and from the venue. 

India's tour Down Under has been beset with Covid-19 related problems. 

Five India players were criticised for seemingly breaching the team's biosecurity bubble by going out to a restaurant. 

And there are still questions about whether the two teams will have to quarantine before the final Test in Brisbane next week.