A fourth worker at the Melbourne Airport quarantine hotel and the spouses of two other staff members returned positive COVID-19 results on Thursday.

The female assistant manager was tested on Wednesday and had been isolating since Monday or Tuesday as the close contact of a fellow Holiday Inn worker.

It followed two men, both in isolation as partners of female food and beverage attendants, testing positive earlier on Thursday.

The Holiday Inn outbreak now stands at 11 and encompasses four hotel workers, two staff spouses, two released guests and a family of three who contracted the virus overseas

Victoria's COVID-19 testing commander Jeroen Weimar said the outbreak had health officials "alert but not alarmed".

"We are right on top of this one," he told reporters.

"We are finding people who tested negative one, two, three days ago."

Genomic sequencing for the outbreak's fifth and sixth cases has confirmed they have the highly infectious UK strain of COVID-19.

Mr Weimar's "working assumption" is the entire cluster has been infected with the more transmissible variant.

Although all of Thursday's three new cases were picked up among 400-500 primary close contacts, Mr Weimar warned the outbreak was "by no means over".

"We are still in the opening quarter of the Holiday Inn outbreak, I'm afraid. We've got a lot more work to do," he said.

"We were here on the Grand Hyatt only a week ago, we've been here with Blackrock ... but this is a bit different.

"It's a UK variant. We don't yet know everything about it, and we have to be on our guard."

The emerging cluster 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.

Western Australia also announced its hard border to Victoria would be extended for at least another seven days.

In addition to strengthened interstate borders, the outbreak has forced the hotel's closure, a pause on a planned increase to Victoria's weekly international travellers cap and multiple health alerts for potential exposure sites.

Authorities suspect a nebuliser, which vaporises medications into a fine mist, may be to blame for the outbreak.

The medical device was not declared in quarantine by one of the family members who has an underlying health condition and was taken to intensive care on Tuesday.

The guest would have been taken to a medi-hotel if they had reported it, COVID-19 Quarantine Victoria Commissioner Emma Cassar said.

Chief Health Officer Brett Sutton said it was possible everyone on that particular floor of the hotel had been exposed to the virus through the air.

There has been separate COVID-19 scares at three Victorian quarantine hotels within the space of a week.

Australian Medical Association president Omar Khorshid said the UK strain had "blown open cracks" in hotel quarantine infection controls.

Victorian minister Jacinta Allan said its program's infection prevention measures were constantly monitored and reviewed when issues arose.

"We're operating this program under the highest, strictest of standards," she told reporters.

Opposition spokesman David Davis said the state government had not learned from shortcomings in the previous iteration of the hotel quarantine program.

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.

Australian Associated Press