Australia, like neighbouring New Zealand, has been lauded for its pandemic response. AFP

Australia reported a record 501 new coronavirus infections Wednesday, nearly four months after the pandemic initially peaked in the country, with authorities warning of a critical new phase in its two biggest cities.

Most of the cases were reported in Victoria state, where authorities have struggled to bring an outbreak in Melbourne under control despite an almost two-week lockdown on Australia's second-biggest city.

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Australia's new coronavirus infections previously peaked on March 28 when 459 cases were reported, according to data compiled by AFP, before the country appeared to bring the virus under control and began easing harsh restrictions.

Australia, like neighbouring New Zealand, has been lauded for its pandemic response.

Medical staff perform a COVID-19 test on a resident in Melbourne. File/AFP

Victoria Premier Daniel Andrews announced 484 new cases and two more deaths in the state Wednesday, bringing the state's total active cases to just over 3,400.

Australia has now recorded almost 13,000 cases of COVID-19 and 128 fatalities in a country of about 25 million.

Melbourne residents will be required to wear masks outside from midnight -- making the city the first place in Australia to mandate face coverings in public -- in a new effort to slow the virus spread.

But Andrews said "further behaviour change" would still be required, after analysis showed almost nine in 10 people diagnosed with coronavirus over the two weeks to Tuesday did not self-isolate between feeling sick and seeking a COVID-19 test.

People in hazardous material overalls are seen outside of a public housing tower in Melbourne, Australia. Reuters

Just over half of people also failed to stay at home between being tested and receiving their results, he added, saying authorities believed insecure work conditions were a key driver of the trend.

Andrews said casual workers across a range of industries, who do not get sick leave provisions, were worried about "feeding their kids, paying their bills" but could apply for a Aus$1,500 ($1,070) hardship payment if they contract the virus.

There are growing fears about the virus spreading through vulnerable populations after outbreaks at several nursing homes while inmates at six prisons have been confined to their cells after a guard tested positive.

The outbreak has left Australia rattled and dampened hopes of a speedy economic recovery.