The COVID outbreak linked to the Melbourne’s Holiday Inn has grown to 13 cases, sparking fears the Victorian capital could again go into lockdown.

Five new cases were confirmed by the Health Department on Thursday, including two announced at 11pm.

“Both are household primary close contacts of previously announced cases,” the department posted on Twitter.

The cases, which have been included in Friday’s official figures, also include a female assistant manager at the hotel and two men, both partners of female workers who earlier tested positive to COVID-19.

One of the men may be linked to Camberwell Grammar School in Melbourne’s east.

In a later tweet, the department said one case was formally reported after midnight. It is included in Thursday’s data as public health actions began then, including the case interview.

“The five cases listed are the cases confirmed publicly yesterday … The Holiday Inn outbreak total remains at 13,” it said.

Tweet from @VicGovDH

Also on Thursday, Victoria’s COVID-19 testing commander Jeroen Weimar said additional exposure sites were likely to emerge.

“That work needs to be done over the coming hours,” he said.

The first of those came early on Friday, with the Brunetti cafe in Terminal 4 at Melbourne Airport listed as an exposure site. It was visited by an infected person on February 9 between 4.45am and 1.15pm.

Anyone who visited the cafe during that time needs to get a COVID-19 test and isolate for 14 days.

• See an updated list of Victorian exposure sites here

As yet, there is no community transmission of the virus from the quarantine cluster.

Mr Weimar said authorities were “right on top” of the outbreak, picking up cases among identified contacts who had tested negative just days earlier.

His “working assumption” is all the cases have been infected with the more transmissible British variant of COVID-19, complicating the containment job for officials.

“This is by no means over,” Mr Weimar said.

“We are still in the opening quarter of the Holiday Inn outbreak, I’m afraid. We’ve got a lot more work to do.”

The rising threat has raised the prospect of Melbourne going into lockdown again, according to the Herald Sun.

Victorian government advisers met on Thursday to consider how this might happen, the news outlet reported on Friday, citing an unnamed source with knowledge of the discussions.

Tweet from @FerrierSteph

The ABC reported that meetings were continuing on Friday morning and “all options are on the table”.

Premier Daniel Andrews will give a briefing later on Friday. No time has yet been confirmed.

Prime Minister Scott Morrison said he had not been briefed on whether the Victorian government was planning another lockdown.

“Hotel quarantine is never 100 per cent fail-safe and to suggest it ever will be is just not realistic,” he told 3AW radio.

“The issue is how you deal with it when it occurs … and the response of Melburnians, once again [has been] tremendous.”

Mr Morrison said Victorian authorities “should be able to get on top of this like other states have”.

“Look at Perth, look at Brisbane, look at Sydney, proportionate short-term responses and they’re back on deck,” he said.

Jane Halton, who’s on the board of the federal government’s National COVID-19 Coordination Commission, said while the number of new cases was worrying, their source had also been identified.

“At the moment we are not seeing reports of any cases that have an unidentified source,” she told Nine’s Today Show.

“I really hope we don’t need to lock down, but everybody right now needs to read that list of what the hotspots are, think about where they have been, get themselves tested.”

The 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.

-with AAP