Of the 20 new cases, 15 have been linked to outbreaks and five remain under investigation. Fourteen were in isolation while infectious.

The entire state was plunged into a seven-day lockdown on Thursday last week, scheduled to end tomorrow night.

However, regional Victoria was granted an early exemption, with the heaviest restrictions eased after just four days.

Melbourne will remain in lockdown until 11.59pm next Thursday, August 19 due to the current level of community transmission of coronavirus in Victoria from the recent New South Wales outbreak.

There are now 116 active locally acquired cases, nearly 300 exposure sites and more than 12,000 active primary close contacts around Greater Melbourne.

"That is the public health advice: there are too many cases - the origins of which are not clear to us - too many unanswered questions and too many mysteries for us to safely come out of lockdown," Premier Daniel Andrews said on Wednesday morning.

Latest data shows Warrnambool is leading the charge with vaccinations in the state, with 2.9 per cent of first doses administered this week.

So far 54.5 per cent of the population over 15 have received a first COVID-19 vaccine dose.

Second dose rates grew to 27.5 per cent, surpassing Melbourne's inner-south at 26.8 per cent and sitting behind Bendigo (28.9 per cent) and Geelong (30.3 per cent).

The news comes as Warrnambool's vaccine rollout steps up to another gear, with people aged between 18 and 39 able to receive an AstraZeneca vaccination at the mass vaccination hub from today.

Premier Andrews urged those eligible for the vaccine to book in today.

"You can you can book with absolute confidence that you can get your first jab, come back twelve weeks later and get your second dose," he said.

"Then you're not going to be in a really long queue in another six or eight weeks time when a lot of other supply gets here.

"If a doctor says it's safe then, as the Chief Health Officer said many times, the best vaccine is the one you can take today.

"That's the biggest contribution you can make to not only your safety, the safety of your family, every family - the jobs of every single Victorian depend on all of us getting vaccinated to 70 per cent."

There are 1700 more close contacts linked to the current outbreak, bringing the number to 13,800.

The Victorian-New South Wales border will be strengthened again and residents of cross-border communities will need a permit to cross the border from 6pm this Friday.

Victoria recorded 20 new local cases of COVID-19 on Tuesday, all of which were linked to known outbreaks and five were in quarantine throughout their infectious period.

There are some 12,000 people self-isolating after coming into contact with an infected case, while the exposure site list grew to almost 300 overnight, including a second housing tower in Flemington.

Of greatest concern to authorities is Caroline Springs Square Shopping Centre, where half of Tuesday's new cases contracted the virus, including two people who may have caught the virus through "fleeting transmission".

Late on Tuesday, health authorities instructed all staff who worked at the shopping centre between August 2 and 5 to immediately get tested and quarantine for 14 days.

Several bus and train routes near the shopping centre have also been listed as exposure sites.

A doctor who works at Melbourne's Royal Children's Hospital was also among the new cases.

However, the hospital said the doctor was not infectious while working at the hospital.

See the exposure sites here.

They worked at private medical rooms next door to the hospital at 48 Flemington Road on Monday.

With daily cases still in the double-digits and the majority not isolating during their infectious period, it's likely lockdown will be extended.

Authorities have refused to rule out an extension to the lockdown, with Health Minister Martin Foley saying decisions were being made on an "hour-by-hour basis".

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