Australia's second largest city Melbourne reinstated COVID-19 restrictions on Tuesday as authorities scrambled to find the missing link in a fresh outbreak that has grown to five cases.
Home gatherings will be limited to five guests, only 30 people allowed at public meetings, and face masks will be mandatory in restaurants, pubs and other indoor venues from 6:00 pm (local time) on Tuesday until June 4.
"This is a responsible step that we need to take to get on top of this outbreak," James Merlino, Victoria state's acting premier, told reporters in Melbourne.
The latest outbreak ends Victoria state's run of zero cases for nearly three months.
Victoria was the hardest-hit state during a second wave late last year, accounting for about 70 per cent of total cases and 90 per cent of deaths in Australia. The state, the country's second most populous, only controlled the outbreak after one of the world's longest and strictest lockdowns.
One new locally acquired case has been reported in Melbourne, Merlino said on Tuesday, a day after four infections were reported in the city.
All five cases belong to one extended family across different households and could be traced back to the variant found in an overseas traveller who returned to Melbourne early this month after completing quarantine in the city of Adelaide.
Authorities, however, said they could not yet find how the latest cases contracted the virus from the overseas traveller.
Thousands have been ordered to self-isolate and undergo COVID-19 tests with health alerts issued for several sites, including one of the largest shopping centres in the country.
One of the cases had a high viral load while he visited some venues prompting authorities to warn Melbourne's five million residents to brace for more positive cases in the next few days.
The fresh outbreak comes as authorities try to ramp up a sluggish national vaccination drive with health experts worried many Australians were delaying getting inoculated because of the country's success in effectively eliminating the virus.
Merlino stressed the importance of getting vaccinated urgently in the wake of the new cases and said the government has plans to expand the vaccination eligibility criteria to rollout doses to more people.
"There are right now millions of Victorians that are eligible to be vaccinated. They shouldn't wait for tomorrow, they shouldn't wait for next week. They should move now and get vaccinated," Merlino said.
Speedy tracing systems, movement curbs and social distancing have largely helped Australia contain COVID-19 outbreaks. It has recorded just over 30,000 cases and 910 deaths since the pandemic began.