Victoria has confirmed another 21 local COVID cases as exposure sites across the state surge to nearly 350.

Of Thursday’s cases, 15 were in quarantine throughout their infectious period.

Four of the daily total are yet to be linked to current outbreaks.

There were also two additional infections acquired interstate on Thursday.

Victorian authorities will give more details in a briefing later in the day.

But the update came as a Qantas flight from Sydney and several paediatric clinics next to Melbourne’s Royal Children’s Hospital joined the list of exposure sites.

Tweet from @VicGovDH

They were added late on Wednesday, after Melbourne’s lockdown was extended by a week. Victoria had 20 new coronavirus cases on Wednesday, including five whose source had not been traced.

Qantas flight QF471 from Sydney to Melbourne on Monday was listed as a ‘tier-one’ exposure site after a COVID patient was on board.

It is unclear if the case was circulating in the community while infectious, given most arrivals from Sydney are required to quarantine.

Ten new tier-two sites have been identified at Melbourne Paediatric Specialists in Parkville, next to the Royal Children’s Hospital, after a healthcare worker attended while infectious on Monday.

Secret Sofa in Cheltenham, in Melbourne’s south-east, is also listed as a tier-one site for August 5 from 11.45am to 12.45pm.

The furniture store is some 50 kilometres from Melbourne’s western suburbs, where the majority of infections and exposure sites are located.

On Wednesday, Premier Daniel Andrews said there was no option but to extend the city’s lockdown, given the mystery cases and the fact six of Wednesday’s 20 cases were not isolating throughout their infectious period.

“There are too many cases, and too many cases the origins of which are not clear to us, too many unanswered questions, too many mysteries for us to safely come out of lockdown now,” he told reporters.

Support for businesses affected by the lockdown extension is expected to be announced on Thursday.

It will be the third round of funding in as many weeks, with $800 million in support already announced.

Meanwhile, 73 people were arrested for gathering in Melbourne’s CBD overnight to protest the lockdown.

-with AAP