The Covid-19 cluster at the Holiday Inn at Melbourne Airport increased to three on Wednesday, which comprises a returnee who left the hotel, a food and beverage worker and a female authorised officer who tested positive for the virus two days ago, reports Xinhua news agency. 

The government agency in charge of Victoria's hotel quarantine system, Quarantine Victoria (CQV) announced the decision to close the hotel on Wednesday morning. 

"The CQV is implementing strict infection prevention and control measures after a hotel quarantine worker, who last worked at the Holiday Inn Melbourne Airport on February 4, tested positive for the virus," it said in a statement. 

"As a highly precautionary measure, the Holiday Inn Melbourne Airport is being closed until further notice for terminal cleaning, and with detailed contact tracing and investigations underway." 

Anyone who spent 15 minutes or more at the hotel between January 27 and February 9 is now considered a primary close contact of the outbreak and will be required to self-isolate for 14 days. 

As many as 48 residents at the Holiday Inn are being transferred to the Pullman Melbourne to quarantine an extended number of days and about 135 staff at the hotel were stood down on Tuesday night and instructed to quarantine for 14 days at home and get re-tested. 

Victorian Premier Daniel Andrews said a plan to increase the state's cap on international arrivals would be put on hold as health authorities race to investigate the new cases. 

The state's Chief Health Officer Brett Sutton said "a working theory" was the three cases were linked to exposure to a medical device called the nebuliser. 

"It vaporises medication or liquid into a fine mist and in that is everything, especially when it is used as medication and someone is infectious or later tests positive, that next up the virus and that mist can be suspended in the air with fine particles. 

"We think the exposures are to that event, this nebuliser whereby the method of a virus was carried out into the corridor and expose the authorised officer, the food and beverage service worker and also the other resident," he said.