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 to the Holiday Inn cluster, 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 February 6.
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 positive" the day after the function, but further analysis of the sample has shown it was 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 positive" 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 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.
Mr Weimar said epidemiologists and specialists were yet to determine whether she is at the start or tail end of her infectious period.
"Serology is being done and we will work out over the next few hours exactly where this individual stands," he said.
The woman has workplace contacts at Alfred Health and authorities plan to undertake preliminary testing within that group.
The two new cases have prompted authorities to add a Woolworths, bakery and two swimming centres to its growing list of exposure sites.
It comes as Victoria's third lockdown forces millions back into the hardship they thought had been left behind with the lengthy restrictions and economic sacrifices of 2020.
Professor Sutton is reassured by 127 negative tests coming back from a cohort of 129 primary close family and social contacts.
But rejected a suggestion the level of negative results showed the snap five-day lockdown was an overreaction.
"This is a high stakes game," Prof Sutton said.
"We cannot afford to be wrong here. We do not want to be in a situation a week from now or two weeks from now where we said 'we'd wish we'd done that'."
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 strain overseas.
One family member, a man now in intensive care, used a medical device for his asthma called a nebuliser in their hotel room, despite them being banned outside of medi-hotels.
The claim was rubbished by CQV chief Emma Cassar, with Mr Foley refusing to be drawn into the war of words on Sunday.
