The health department confirmed the state recorded six cases in the 24 hours to Friday morning, two of which were already reported by authorities on Thursday.

All six cases are linked to previously reported infections of the Delta variant and have been in the community while infectious.

Some 29,631 test results were processed on Thursday and 18,901 Victorians were vaccinated.

The statewide lockdown began at 8pm on Thursday. The same rules that applied during last month's lockdown will be reimposed, including the five reasons to leave home, the five-kilometre travel limit for exercise and shopping and compulsory masks indoors and outdoors.

Premier Daniel Andrews said the decision to lock down was "incredibly painful" but there was no alternative.

"The alternative is not to be locked down for seven days, it's being locked down for seven weeks or more, locked down until we get to 80 per cent vaccination and that may not happen until Christmas time," he said.

Further financial support for businesses is expected to be announced on Friday, with those that applied for grants and income support during the last lockdown to receive payments again.

Fifteen protesters were arrested after they rallied against the lockdown in Melbourne's CBD overnight.

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

Australian Associated Press