A second worker at a Melbourne Holiday Inn quarantine site and a returned traveller who has already completed quarantine in Australia have tested positive to Covid-19.
The quarantine returned traveller had already completed quarantine and left the facility on Sunday.
She got tested after an initial hotel quarantine staff member tested positive on Sunday to the virus - found later to be the more infectious UK strain.
Her result came out as positive on Tuesday, despite testing negative a number of times during her 14-day quarantine.
Late on Tuesday, Australia's Health Minister Martin Foley also confirmed a food and beverage worker at the quarantine hotel has also tested positive.
The Department of Health and Human Services says past residents of the Holiday Inn who completed their quarantine period on Sunday must now isolate for 14 days.
"We are in contact with this group. This will come as difficult news to them, but we are being as cautious as possible in our response to this outbreak," the department tweeted on Tuesday afternoon.
The department says a public health response is underway, and the individual is being interviewed.
"Early indications are that the individual has not left her home since exiting Hotel Quarantine on 7 February, other than to obtain a test in a Covid-safe setting," the department tweeted on Tuesday afternoon.
Updates to any exposure sites, primary close contacts and other information will be provided as soon as it's known.
Premier Daniel Andrews earlier on Tuesday said there was a "working theory" and "very significant concerns" regarding aerosol transmission of the highly infectious strains within hotels.
"They do present us with a very significant challenge," the Premier said of the highly infectious variants.
"Out of the Holiday Inn, the CCTV footage review, from all the other checking that we do, there's no obvious infection prevention and control breach.
"It's not like someone's obviously done the wrong thing, and that's a significant concern to us."
Health authorities are also hoping genomic sequencing will help provide some insights into how the first worker at the Holiday Inn, who wore full personal protective equipment, including a surgical mask and face shield, became infected.
At least 136 hotel quarantine workers at the Holiday Inn had been identified as primary close contacts and were told to isolate and get tested before the second worker tested positive.
A number of potential public exposure sites have also been identified in Melbourne's northwest suburbs with anyone who has visited the "tier 1" sites during the times listed is required to immediately isolate, get tested and remain isolated for 14 days from the time they were at the site.
Testing capacity near exposure sites in Melbourne's northwest is also being enhanced.
It comes after another hotel quarantine worker – a 26-year-old man from Noble Park based at the Australian Open quarantine program – also returned a positive Covid-19 test result last Wednesday.
Genomic sequencing confirmed he had also been infected with the UK strain.
The health department said more than 61,000 Victorians had been tested since Thursday morning.