Australia's Victoria state reported fewer new local COVID-19 cases on Friday, the first day of a one-week hard lockdown imposed to contain a highly-infectious outbreak which authorities said could become uncontrollable.

Four new locally acquired cases were reported in the last 24 hours, down from 12 a day earlier, taking the total infections in the latest cluster to 30 but officials urged people to remain cautious and follow lockdown rules.

"We are very, very early in this ... community transmission is still expected to occur," Victoria state Chief Health Officer Brett Sutton told reporters in the state capital Melbourne.

Victoria, Australia's second most populous state, entered into a lockdown on Thursday night, due to run until June 3, forcing its near seven million residents to remain home except for essential business

People are allowed to leave their homes only for five reasons - essential work, healthcare, grocery shopping, exercise or to get a coronavirus vaccination. Masks must be worn at all times when they leave homes.

The latest outbreak was caused by a traveller, who left hotel quarantine in South Australia state after testing negative, but later tested positive in Melbourne.

More than 15,000 primary and second contacts have been identified, a rise of 5,000 in 24 hours, while authorities listed more than a hundred virus-exposed locations.

Medical officials have said the latest COVID-19 variant, first detected in India, was likely to be more virulent than the original strains, taking one day to infect another person compared with earlier strains which can take about five or six.

In announcing the lockdown on Thursday, they warned the outbreak may become uncontrollable.

The outbreak has spurred a surge in people getting tested and vaccinated in Victoria. More than 17,000 vaccinations were completed on Thursday and Victoria recorded more than 47,000 tests, its largest one day number.

Australia has struggled with a slow vaccine roll out and rising vaccine hesitancy because of the country's COVID-19 success in virtually eliminating the virus. The country has reported zero local cases for 94 days this year while it has remained in low single digits on most other days.

Australia has effectively contained all past outbreaks through speedy contact tracing, snap lockdowns and strict regional border controls, helping keep its COVID-19 numbers relatively low, with just over 30,060 cases and 910 deaths.

But its nationwide vaccination drive missed its initial dosage targets amid supply issues and a policy change allowing those younger than 50 to receive the Pfizer vaccine.