Why Melbourne's lockdown could be extended AGAIN as 21 new cases are recorded and furious business bosses lash Dan Andrews' over his 'yo-yo restrictions that are never short or sharp

Plans are already underway to plunge long-suffering Melburnians into a third week of lockdown, just hours after stay at home orders were extended.

Victoria's renewed outbreak, which prompted a sixth lockdown last week, added another 21 cases on Thursday including four mystery infections.

By next Thursday, Melburnians will have spent 200 days holed up at home, after a disastrous 116-day lockdown last year.

Health officials are already in discussions about yet another extension, just hours after Mr Andrews announced on Wednesday the city would remain lockdown for another week.

Of the new cases, 17 have were linked to the outbreak with four still under investigation and 15 in quarantine throughout their infectious periods.

 Long-suffering Melburnians (pictured on Wednesday) could face a third week of lockdown

They were diagnosed from 45,408 tests and 25,418 vaccine doses were administered.

An expert close to the Victorian government said 'everything would have to work out perfectly' for lockdown to not be extended once again, The Herald Sun reported. 

Australian Industry Group chief executive Innes Willox said Melbourne is in a hard place with businesses tired of being fed false hope.

'The cold hard reality hit in Melbourne that there is no such thing as short sharp lockdown,' Mr Willox told the Today show on Thursday.

'We just roll into longer lockdowns every time. Every lockdown is going to be about three weeks at the minimum.'

'Melbourne and Victoria is now facing just continual yo-yos of lockdown. That's playing havoc with planning, with investment and it's really start to sap what confidence that was there.

Melbourne's sixth lockdown in 18 months was scheduled to end 8pm Thursday, but has now been extended until 11.59pm on August 19.

By Thursday next week, Melburnians will have spent their 200th day holed up at home since the pandemic began 18 months ago

Regional Victoria was lifted was lockdown earlier this cases after no cases from the latest outbreak.

Melbourne epidemiologist Professor Nancy Baxter said it was 'unlikely' a seven-day extension would be enough.

'Those 14 cases in isolation gave them a ray of hope, but I still think everything would need to be perfect to come out in a week,' she  told the Herald Sun.

'There's not a zero chance that we could get out on time, but there are a number of things that concern me, and we know that you don't have a margin of error with Delta.'

Victoria recorded 20 new cases on Wednesday, six of them infectious in the community.  

Mr Andrews fears the city would see a huge spike in cases similar to the current outbreak in Sydney if stay-at-home orders were lifted this week.

Melbourne is in the middle of its fourth lockdown in 2021, which could be extended a second time next week if cases continue to grow. Pictured is a deserted CBD

'This is very challenging, I know, for every single Victorian who would like to be going about their business, they would like to be open and have a degree of freedom that is simply not possible because of this Delta variant,' he told reporters on Wednesday.

'If we were to open, we would see cases akin to what's happening sadly Sydney right now.' 

Fifteen of the Wednesday's cases have been linked to current outbreaks while five are still being investigated as the state's exposure list site grew to more than 350 venues and transport routes.

These mystery cases are cause for concern, the Premier warned.

'I would love to be standing here announcing that we are coming out of lockdown this Thursday that but that is simply not possible,' Mr Andrews said.

'There are 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 continued: 'It is more than containing cases, to have 14 of those 20 locked away in iso, no risk to public health, as I said earlier in the week, that trend would grow.

'We started with one case, then we moved to four or five, now we're at 14.'

One expert said 'everything would have to work out perfectly' for Melbourne's lockdown to not be extended once again next week