Australia on Thursday reported a record number of new coronavirus infections and its deadliest day of the pandemic so far following a spike in cases at elderly care homes.

Days after authorities expressed hope that a Melbourne lockdown -- now in its third week -- was bringing persistent outbreaks under control, the surge is a potent warning that the country's initial success in managing COVID-19 can quickly unravel.

Thirteen deaths and 723 positive tests were reported in the southeastern state of Victoria alone, well beyond the previous nationwide record of 549 cases set on Monday.

Premier Daniel Andrews indicated the leap, was in part, linked to a surge in cases in aged care homes.

Most of those who died were aged in their 70s-90s.

At one of the worst-hit facilities, Epping Gardens, an ambulance was seen Wednesday taking away the body of one of the deceased residents.

Health workers rolled other masked residents on stretchers to waiting ambulances for transfer to hospital.

Melbourne has new infections
Melbourne has had thousands of new infections since mid-June. Photo: AFP / William WEST

The rest of the country remained on high alert as three new cases were reported in Queensland and several schools in Sydney were shuttered overnight amid growing fears that the country's largest city may also be tipping toward an uncontrollable outbreak.

New South Wales reported 18 new cases, roughly in line with previous days.

Australia has been one of the most successful countries in containing the pandemic, with most states and territories reporting few or no new cases in recent weeks.

But Melbourne has had thousands of new infections since mid-June, an outbreak initially blamed on security bungles at hotels where returning international travellers were under mandatory quarantine.

The government has now extended some of Melbourne's lockdown measures to surrounding cities and deployed disaster-relief teams to elderly care homes in the city.

Those teams -- which usually tackle earthquakes and other disasters overseas -- include doctors, nurses, paramedics and logistics experts.

Australia has recorded over 16,000 COVID-19 cases and 189 deaths, in a population of 25 million.

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