Authorities are yet to rule out an extension to Victoria's five-day "circuit breaker" lockdown, as the state records one new locally acquired case of COVID-19.
Premier Daniel Andrews on Monday confirmed the case is a woman who attended a function with an infectious hotel quarantine worker on 6 February.
The woman is asymptomatic and was tested four times at the weekend, returning both negative and "weak positive" results.
Her three-year-old child tested positive on Sunday.
"Given her exposure and the variability of those results, the public health team have taken the most conservative approach and have deemed her a positive case," Mr Andrews told reporters.
A psychiatric unit at The Alfred hospital and psychiatric wards at Broadmeadows Hospital and the Northern Hospital in Epping, where the woman worked, have been locked down.
About 150 of the woman's close contacts across the three hospitals, including a "small number" of patients, have been tested and will be required to self-isolate for 14 days.
"This has been a very rapid response and one that is filled with an abundance of caution, but that is exactly the approach that we ought to take," Mr Andrews said.
Victoria's COVID-19 Testing Commander Jeroen Weimar said the woman's child attended Glenroy Central Kinder and Goodstart Early Learning Centre in Glenroy for three days last week.
More than 100 close contacts have been identified and a dedicated testing site has been set up at the larger of the two facilities.
Chief Health Officer Brett Sutton stressed the risk of transmission among children is low.
"Everyone who's been in the early learning or childcare settings or had children with this three-year-old should be reassured by that (and) also by the fact that young children have a very low risk of significant illness," he said.
The mother and child, as well as a woman aged in her 50s, contracted the virus after attending a family function on Sydney Road in Coburg on 6 February.
The function was attended by 38 people including a worker from the Holiday Inn quarantine hotel at Melbourne Airport, who had returned a negative test result on 7 February.
The venue was not listed as an exposure site until 12 February, two days after the hotel quarantine worker eventually tested positive.
A review of the worker's earlier test result found it was a "false negative".
A total of 17 people linked to the Melbourne Airport Holiday Inn have now tested positive for COVID-19.
Mr Andrews said some of the cases were not in the original "net" cast by authorities.
"This just confirms the advice from the CHO to me and cabinet, to have this circuit-breaker, was the right call and remains the right call," he said.
The premier said he was not in a position to confirm the lockdown would end as planned on Wednesday.
"I think we are well placed. However, I've never been one to try and make bold predictions. We just have to take this one hour at a time, one day at a time," Mr Andrews said.
"This is a promising start, these last three days, and I am proud of all Victorians for the hard work that they've put in."
Both Mr Andrews and Prof Sutton indicated they wanted to see more data before making a decision on ending the lockdown.
"Every single case has been linked. Those immediate social and household close contact testing negative, that's all terrific news," Prof Sutton said.
"I am always aware that we can have a day when something comes completely out of left-field so I hope that doesn't happen."
A returned traveller in hotel quarantine also tested positive to COVID-19 on Monday, bringing the total number of active cases in the state to 21.
More than 25,000 Victorians were tested for the virus on Sunday, the highest single-day total for more than a month.
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