A false-negative test result from a Holiday Inn hotel worker put Victorian authorities off the scent of a family function where COVID-19 spread into the community.
A three-year-old child and a woman in her 50s were the latest locally acquired infections reported on Sunday, the second day of Victoria's five-day lockdown.
Health Minister Martin Foley confirmed both are linked , bringing its total number of cases to 16.
The pair, who are from separate households, were at the private family function on Sydney Road, Coburg on 6 February.
It was attended by 38 people including a COVID-19 Quarantine Victoria worker at the Melbourne Airport hotel who tested positive on Wednesday.
"It is a notable event and that is now the lead line of our investigation," COVID-19 testing chief Jeroen Weimar told reporters on Sunday.
The female worker returned a "false negative" the day after the function, but further analysis of the sample has shown it to be a "weak positive".
Chief Health Officer Brett Sutton said the Coburg venue wasn't initially within contact tracers' scope due to the worker's "false negative" result and defended the time taken to identify it as an exposure site.
"To be a weak positive, but to be infectious enough to actually cause infection in other people at an event is very unusual," he said.
Since then, the worker's partner and housemate, the three-year-old child and the woman in her 50s have tested positive.
Of the 38 people now in isolation, 30 people have so far tested negative.
But the young child's mother could be another possible case in waiting after returning three conflicting test results over the past 24 hours.
"Serology is being done and we will work out over the next few hours exactly where this individual stands," Mr Weimar said.
The two new cases have prompted authorities to add a Woolworths, bakery and two swimming centres to its growing list of exposure sites.
Despite the reassuring rate of negative results from Saturday's tally of 21,475 tests, Professor Sutton rejected the snap five-day lockdown was an overreaction.
"This is a high stakes game," he said.
"We cannot afford to be wrong here."
There are 940 primary close contacts associated with the Holiday Inn outbreak who are isolating for 14 days and being tested.
Mr Foley said it was too early to tell whether the "circuit breaker" lockdown had been successful in containing the 16-case cluster.
It has been traced back to a family of three who quarantined at the Holiday Inn and are suspected to have caught the more infectious UK variant overseas.
One family member, a man now in intensive care, used a nebuliser in their hotel room for his asthma, despite the medical device being banned outside of medi-hotels.
He claims to have declared the nebuliser to medical staff at the hotel.
That suggestion , with Mr Foley refusing to be drawn into the controversy on Sunday.
Meanwhile, authorities have confirmed Greek tennis player Michail Pervolarakis tested negative before flying out of Melbourne international airport on Tuesday and returning a positive result after his arrival in South Africa.
Mr Foley said the world no. 463, who played in the ATP Cup at Melbourne Park as part of the Australian Open lead-up, was advised by South African officials that he'd picked up the virus during his stopover in Doha.
However, Professor Sutton suspects the case is a false positive.
"I don't think it's a real result," he said.
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