The state recorded 20 new COVID-19 cases on Wednesday, of which 15 are linked to known outbreaks, but the origins of five are a mystery.

Premier Daniel Andrews said there was no option but to extend the lockdown, given the mystery cases and the fact six 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.

Mr Andrews said if the state were to open with the Delta variant circulating in the community, there would be a high number of cases in a short space of time.

"We would see cases akin to what's happening, tragically, in Sydney right now," he said.

The lockdown, the state's sixth, was due to end on Thursday but will be extended until 11.59pm on August 19.

Regional Victoria will remain out of lockdown, but those living on the Victoria-NSW border will need a permit to travel between states from 6pm Friday.

Spread of the virus in Caroline Springs remains the main concern for authorities, closely followed by the local government area of Melton, where three of the mystery cases live.

The remaining two mystery cases are a healthcare worker, who did a shift at the eye clinic at the Royal Children's Hospital on Monday, and his son, a student at St Michael's Primary School in North Melbourne.

There are now 100 cases linked to the current outbreak, 13,800 close contacts isolating and about 300 exposure sites listed.

Despite the lockdown extension, Chief Health Officer Brett Sutton is confident authorities are "at the heels" of the virus.

He urged anyone with symptoms to get tested, however testing sites in Melbourne's west are at capacity, despite extra lanes and opening hours.

Mr Andrews said support for businesses affected by the lockdown extension would be announced on Thursday, while he would have "more to say" about expanding the vaccination program to meet demand soon.

Thousands of Victorians aged between 18 and 40 have received the AstraZeneca vaccine since eligibility was expanded on Monday.

Opposition Leader Michael O'Brien urged the government to ease restrictions for parts of Melbourne that have no cases or exposure sites.

"Why is everyone in Melbourne, every one of those five million people in Melbourne, subject to this lockdown?" he asked.

Mr Andrew's press conference on Wednesday was interrupted by a man who claimed he had a letter for the premier.

He was tackled by protective services officers and was photographed by AAP being stretchered into an ambulance. .

"We don't know if it's even about COVID. It's somebody who I think has wanted to speak to me for some time. I don't know who it is," Mr Andrews said.

Victoria Police said Protective Service Officers apprehended the 51-year-old Brunswick man at the press conference and he was taken to hospital for an assessment.

For exposure sites visit www.coronavirus.vic.gov.au/exposure-sites

Australian Associated Press