Premier Daniel Andrews confirmed Victoria's new locally acquired case on Monday is a woman who attended a family function with a COVID-infected hotel quarantine worker on February 6.

The woman is asymptomatic and was tested four times at the weekend, returning both negative and "weak positive" results.

"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.

The premier said the woman worked in a psychiatric unit at the Alfred Hospital and on psychiatric wards at the Northern Hospital in Broadmeadows, which is run by Royal Melbourne Hospital.

"Those services have had those wards locked down. Staff, all those that she may have come in contact with, they are all isolating and have been tested," Mr Andrews said.

"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."

Victoria's COVID-19 Testing Commander Jereon Weimar said 150 primary close contacts across the two hospitals have been identified.

"The majority are staff at those facilities, a very small number of patients," he said.

The woman is the mother of a three-year-old who tested positive to the virus on Sunday.

Mr Weirmar said the child attended Glenroy Central Kinder and Goodstart Early Learning Centre in Glenroy over three days last week. About 101 primary close contacts have been identified.

The woman and child, as well as another woman aged in her 50s, contracted the virus after attending a family function on Sydney Road in Coburg on February 6.

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 February 7.

The venue was not listed as an exposure site until February 12, two days after the hotel quarantine worker eventually tested positive.

A review of the worker's February 7 test result found it was a "false negative".

Some 17 people linked to the Melbourne Airport Holiday Inn have tested positive for COVID-19.

Burnet Institute epidemiologist Michael Toole said the Coburg function does not appear to be a super-spreading event.

"Those two people at the Coburg site were infected about a week ago or were exposed a week ago. We haven't had any other cases outside of close contacts, so that's looking good," he told 3AW radio.

Professor Toole said he supported the five-day precautionary lockdown.

"It was shown in Brisbane and Perth that this approach does work," he said.

"It looks like there hasn't been extensive spread here."

Mr Andrews would not be drawn on whether Victorians could be confident the "circuit breaker" lockdown would not be extended beyond Wednesday.

"We are well placed right now, but right now is too early to be definitive about Wednesday evening," he said.

The Health Department confirmed there were two new cases on Monday, the other being a returned traveller in hotel quarantine.

It brings 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.

Australian Associated Press