Victoria's Holiday Inn hotel quarantine outbreak has hit double figures, with two household contacts of infected workers testing positive for COVID-19.
The Victorian Department of Health on Thursday reported the two new infections were linked to the now-closed hotel at Melbourne Airport.
They are both household primary close contacts of previously confirmed COVID-positive Holiday Inn workers and take the total number of cases linked to the cluster to 10.
Another two cases, a third hotel worker and second former resident, were officially added to Victoria's tally earlier on Thursday.
The outbreak now encompasses three workers, two primary household close contacts, two released guests and a family of three who contracted the virus overseas.
It has prompted several states to tighten their borders to travellers from Greater Melbourne.
South Australia locked out travellers from the Victorian capital at midnight on Thursday, while Queensland will bar entry to visitors of the city's exposure sites from 1am on Saturday.
It comes as a new health alert was issued for an entire shopping centre in Melbourne's northwest late on Wednesday.
Anyone who visited the Sunbury Square Shopping Centre on February 5 between 3.40pm and 4.30pm is urged to get tested and isolate until they receive a negative result.
They described the move as a "precautionary approach" after a hotel quarantine worker visited multiple stores while infectious.
The advice contrasts with that of other exposure sites, including two banks at Glen Waverley in Melbourne's southeast, where people who attended must get tested and isolate for 14 days regardless of the result.
Despite a case visiting an osteopathic clinic on February 5, health officials have decided not to list it as a potential exposure site as record-keeping had allowed all contacts to be followed up.
"All patients and staff who were at the clinic at the time have been contacted and tested and, where required, have gone into isolation for 14 days," a Department of Health spokesman told AAP in a statement on Thursday.
In addition to tightening interstate borders, the outbreak has forced the hotel's closure until further notice and a pause on Victoria's weekly increase for international arrivals which was originally planned for Monday.
Authorities suspect a nebuliser, which vaporises medications or liquids into a fine mist, may be to blame for the outbreak.
The medical device was not declared by one of the family members in quarantine, who has an underlying health condition and was taken to intensive care on Tuesday.
COVID-19 Quarantine Victoria Commissioner Emma Cassar said the guest would have been taken to a medi-hotel if they had reported it.
More than 135 hotel staff were stood down and told to get tested and isolate at home for 14 days, while 48 guests were moved to the Pullman Melbourne to quarantine for at least another three days.
Chief Health Officer Brett Sutton said it was possible everyone on that hotel floor had been exposed to the virus through the air and warned more cases would likely be unearthed.
There have been cases of COVID-19 transmission across three Victorian quarantine hotels within a week, with three confirmed to be the more infectious UK strain.
Australian Medical Association president Omar Khorshid said the UK strain had "blown open cracks" in hotel quarantine infection controls.
Deputy Opposition Leader David Davis said the Victorian government had not learned from shortcomings in the previous iteration of the hotel quarantine program.
"It's clear there are deficiencies," he said.
But Prime Minister Scott Morrison, in Melbourne's outer southeast on Thursday to spruik the federal government's HomeBuilder plan, is not worried about the outbreak.
"I wouldn't be here if I wasn't confident," he told reporters at Officer.
.png?w=600)
