The Holiday Inn quarantine hotel at Melbourne Airport that is the origin of three COVID cases has been closed as a probe into the outbreak gathers pace.

The development came as Victorian authorities revealed a simple piece of medical equipment was likely to blame for the growing outbreak.

Victorian chief health officer Brett Sutton said the handover of a nebuliser – a device that vaporises medication or liquid into a fine mist – was considered the most likely point of transfer of the virus.

“That makes sense in terms of the geography and it makes sense in terms of the exposure time,” he said on Wednesday.

“We need to be acutely aware of the possibility of everyone who was on that floor, in particular, being exposed to that.”

Just hours earlier, the government agency responsible for Victoria’s hotel quarantine, COVID-19 Quarantine Victoria, said anyone who had visited the hotel for any reason during the exposure period from January 27-February 9 was considered a primary close contact and would need to quarantine.

Tweet from @VicGovDH

Premier Daniel Andrews said the hotel would be closed for a terminal clean, which can’t be done with guests present.

He said the mutant cases of the virus emerging in Victoria and across Australia were “a very significant cause of concern”.

“These are, as we know, highly infectious, hyper-infectious, really, and it is a very significant cause for concern, not just for us but I think for every government and public health officials across the country,” he said.

“This is a wicked enemy, made more challenging by the fact that it is changing, it is a moving target, and that really does cause significant concern for all of us.”

Two workers at the Holiday Inn have been confirmed to have the coronavirus this week, as well as a guest who recently completed a fortnight’s quarantine.

Late on Tuesday, Victorian authorities issued notices of new virus exposure sites after the hotel COVID-19 cluster grew to three.

CQV said the hotel was being closed “as a highly precautionary measure”.

About 135 Holiday Inn staff were stood down on Tuesday night. They were told to get tested and then isolate at home for 14 days, bringing the total number of staff isolating to 220.

Tweet from @Nic_Asher

More than 950 hotel quarantine staff across Victoria are isolating after recent cases of transmission at the Holiday Inn, Grand Hyatt and Park Royal quarantine hotels.

Forty-eight guests of the Holiday Inn considered primary close contacts were being transferred to the Pullman Melbourne on Wednesday morning. The clock on their 14-day quarantine period will be reset back to day one.

“That will be very challenging for those people, but we simply cannot run the risk of this variant, this hyper-infectious strain of COVID-19 getting out into the community,” Mr Andrews said.

The returned traveller had tested negative several times during her stay, which ended on Sunday. She got tested again on Monday after learning of the outbreak.

The woman did not leave home other than to get tested and only one primary close contact has been identified so far.

The food and beverage worker worked on the same floor as the returned traveller and was identified as a close contact of the positive authorised officer who was the first case to emerge from the hotel.

On Tuesday, Professor Sutton said the three Holiday Inn cases were likely linked to a floor of the hotel where known COVID-positive guests were staying.

That includes a family of three, one of whom is in intensive care. Mr Andrews said that person was “dealing with some very significant health challenges”.

As a result of the outbreak, Victoria will also pause a planned increase in the numbers of returned travellers it takes from overseas. That was due from Monday.

Mr Andrews said he had informed the federal government of the decision.

“We are not taking any risks that would see us have to get to a situation where we ask Victorians to deal with other restrictions,” he said.

“Victorians have given a lot, and I’d just won’t run the risk until we know and understand exactly the nature of the challenge that this changing virus presents to us.”

He said Australia’s quarantine and the public health response to the virus had to keep changing “because the enemy we are confronting is different than it was a month ago or six months ago”.

Virus testing is also being boosted across Melbourne’s north and west on Wednesday to cope with an expected influx of people demanding tests.

-with AAP