Another worker and former guest of Melbourne's Holiday Inn quarantine hotel have tested positive for COVID-19, bringing the total number of cases linked to the outbreak to eight.
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The health department on Wednesday confirmed two new infections had been linked to the hotel at Melbourne Airport.
They include a worker and past resident who left the hotel on Sunday, the same day as an already known COVID-positive guest departed.
The two new infections mean the outbreak now encompasses three workers, two released guests and a family of three no longer staying at the hotel.
Authorities believe a medical device in the room of the family could be to blame for the cluster.
The outbreak has forced the hotel's closure, while plans to increase the state's weekly cap on international arrivals from 1120 to 1310 from next week have been put on hold.
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Victoria's Chief Health Officer Brett Sutton said authorities believe the cases are linked to the use of a nebuliser, a device that vaporises medication or liquid into a fine mist.
"If that's breathed in and someone is infectious or later tests positive, then that picks up the virus and then that mist can be suspended in the air with very fine aerosolised particles," he told reporters on Wednesday.
The nebuliser was used by a Holiday Inn guest taken to intensive care on Tuesday.
SEE THE UPDATED COVID-19 EXPOSURE SITES AT THE BOTTOM OF THE ARTICLE
The guest, who has an underlying health condition, and their two family members contracted the virus overseas.
Professor Sutton said it was possible everyone on that floor of the hotel has been exposed to the virus.
"The risk with an aerosolised virus is very substantial and so I think we should expect more cases," he said.
COVID-19 Quarantine Victoria Commissioner Emma Cassar said the guest had not reported the nebuliser when arriving at the hotel, and would have been taken to a medi-hotel if they had.
"I'm sure they didn't believe they were doing the wrong thing, but we are taking extra steps to make sure those machines are not in the hotel," Premier Daniel Andrews said.
Mr Andrews said the infected workers and guest's close contacts have been tested and a number of exposure sites have been identified.
He described the emerging mutant strains of COVID-19 as a "very significant cause for concern".
"These hyper infectious strains are proving very difficult to contain and that's a real challenge," Mr Andrews said.
"We need to redouble our efforts to do more and to respond to that unique challenge."
About 135 staff at the hotel were stood down on Tuesday night and told to get tested and isolate at home for 14 days, bringing the total number of staff isolating to 220.
Forty-eight guests of the hotel considered primary close contacts will be transferred to the Pullman Melbourne.
Any guests who were due to leave quarantine in the next three days will be required to stay at least another three days.
In January, Brisbane's Hotel Grand Chancellor was evacuated after a cleaner tested positive to the UK strain. Returned travellers were moved to another hotel to restart their 14-day quarantine period.
There have been seven locally acquired cases of COVID-19 across three Victorian quarantine hotels within a week, with three confirmed to be the more infectious UK strain.
National AMA president Omar Khorshid said the UK strain had "blown open cracks" in hotel quarantine infection controls, exposing the need for urgent action.
"The virus has now escaped hotel quarantine arrangements in most states, and we are incredibly lucky to have not yet seen a mass outbreak," Dr Khorshid said.
More than 950 hotel quarantine staff across the state are isolating.
NEW COVID-19 CASE EXPOSURE SITES (UPDATED)
Friday, February 5
* PJ's Pet Warehouse, Sunbury, 3.37pm - 4.10pm
* Bakers Delight, Sunbury Square Shopping Centre, 3.40pm - 4.15pm
* Aldente Deli, Sunbury Square Shopping Centre, 3.45pm - 4.23pm
* Sushi Sushi, Sunbury Square Shopping Centre, 3.53pm - 4.28pm
* Asian Star, Sunbury Square Shopping Centre, 3.57pm - 4.30pm
Saturday, February 6
* Cellarbrations, Sunbury, 6.17pm - 7.02pm
* Sunny Life Massage, Sunbury Square Shopping Centre, 4.30 - 6.30pm
Saturday, February 6
* Off Ya Tree Watergardens, Taylors Lakes, 1pm - 1.52pm
Sunday, February 7
* Cellarbrations, Sunbury, 5.44pm - 6.19pm
-AAP