Victoria has recorded 21 new locally acquired COVID-19 cases, including four mystery infections.

The health department on Thursday confirmed 17 of the new cases are linked to known outbreaks, while 15 were in quarantine during their infectious periods.

Some 45,408 tests were processed in the 24 hours to Thursday morning, while 25,418 Victorians were vaccinated at state-run hubs. 

It comes after Melbourne’s lockdown was extended by a week, in part due to the discovery of five cases of unknown origin, also known as mystery cases, on Wednesday. 

There are more than 13,800 close contacts isolating, while the number of exposure sites has surged to almost 350. 

Qantas flight QF471 from Sydney to Melbourne on Monday was listed as a ‘tier-one’ exposure site after a COVID-19 case was on board.

It is unclear if the case was 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 southeast, is also listed as a tier-one site for August 5 from 11.45am to 12.45pm. 

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

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. 

AAP