Victorian health authorities are combing through CCTV and potentially thousands of AFL fans will be ordered into self-isolation after a person with COVID-19 went to a match at the Melbourne Cricket Ground at the weekend.
The Victorian outbreak had already grown to nine and impacted venues outside of its origin Melbourne suburb of the City of Whittlesea when the MCG link was revealed on Tuesday, just before midnight.
The positive case sat in zone four, level one, of the Great Southern Stand, at the Punt Road end of the ground, to watch Collingwood's nail-biting loss to Port Adelaide on Sunday, the Department of Health said.
READ MORE: How states and territories are reacting to Victoria's new COVID-19 cases
https://twitter.com/VicGovDH/status/1397188001382518792
Fans seated in bays M1 to M16 will be contacted and told to get tested and isolate but that advice could be extended to more parts of the stadium after security footage is reviewed.
The 100,000-seat venue was less than a quarter full for the Power's one-point win, according to Austadiums, but thousands of fans may still have been sitting in the affected section.
It's not clear which of the nine cases identified so far watched the Pies-Power clash but the potential spreading of the virus at the MCG poses a major setback for Victorian authorities racing to keep the cluster from growing.
Five new infections announced on Tuesday came after a testing blitz in Melbourne and added to a rapidly growing list of exposure sites that spread from Epping in the city's outer north to the CBD.
McDonald's in Clifton Hill and Bamboo House in the CBD were added to the list of more than 20 tier-one sites on Tuesday, the first such sites outside of the City of Whittlesea.
Other notable tier-one locations — sparking a 14-day quarantine and immediate testing for anyone linked to them — include supermarkets, restaurants and a swim school.
READ MORE: Collingwood AFL match linked to Melbourne COVID-19 outbreak
Sweeping restrictions came into force across greater Melbourne at 6pm on Tuesday in an attempt to contain the outbreak, limiting gatherings to five people in a private home and 30 people in public.
Masks are mandatory indoors for anyone older than 12 without an exemption until the rules are reassessed on June 4.
Schools and workplaces remain open and density requirements at restaurants and bars remain unchanged but the government is seeking advice on weddings and sporting and cultural events.
READ MORE: Testing centres in Melbourne overwhelmed as cases rise
Victoria's Chief Health Officer Brett Sutton was concerned a man identified as the fifth case in the latest cluster may have been infectious since at least May 17.
Contact tracers have established the man met with the first identified case for a business transaction on May 18.
Acting Victorian Premier James Merlino said genomic sequencing for the northern suburbs outbreak showed it was "closely linked" to the Wollert cluster from earlier this month, which originated in South Australia hotel quarantine.
Victorians who live in Greater Melbourne and travel to regional Victoria can continue to do so, but the restrictions will travel with them.
Mr Merlino said the new restrictions were a necessary and "responsible step" in order for the state to stamp out the growing outbreak.
"This is about giving our contact tracers the time they need to track this matter down and get on top of it," he said.
Professor Sutton said the changing situation was a solid reminder that "millions" of Victorians who had not been vaccinated should go and get their jabs.
"We have enough supply," he said.