MELBOURNE (REUTERS) - Sydney will get 50,000 more doses of the Pfizer vaccine this week to battle its worsening Covid-19 outbreak, Canberra said on Saturday (July 24), reversing a rebuff by the Australian government and other states the previous day of a plea for more supplies.

Australia reported 176 new locally acquired Covid-19 cases on Saturday, a third consecutive daily record with nearly all cases in the state of New South Wales, centred in the country's largest city, Sydney.

Officials fear the outbreak could jeopardise the rest of the country.

Defying a statewide stay-at-home order, however, 3,500 mostly maskless protesters clashed with police in downtown Sydney, decrying a month-long lockdown in what the state police minister said was the best case study of a superspreader event since the start of the pandemic.

"You don't have to be an epidemiologist to work out that if this is a superspreader event, we can forget about lifting restrictions next week," New South Wales Police Minister David Elliott told reporters.

Police arrested 57 people with more under investigation, facing hefty fines and penalties.

There were protests in Melbourne and Adelaide, also under lockdown, and in Brisbane, where there are no restrictions.

Police arrested six people in Melbourne, including one for assaulting an emergency worker, and fined dozens of others for failing to comply with orders to stay within 5 km of home.

New South Wales Health Minister Brad Hazzard stressed the state's urgent need for extra vaccine supply, adding that with so many people flouting the stay-at-home orders it was the only way to stop transmission of the virus.

"At the moment it's like fighting a war with both arms behind your back," Hazzard told reporters.

New South Wales reported 163 locally acquired cases of Covid-19, up from 136 the previous day, with 37 patients in intensive care, in the outbreak centred in Sydney.

Mr Hazzard renewed a call for other states to give up some Pfizer vaccines so that younger people in Sydney's hot spots could be vaccinated, reminding neighbouring states about the help New South Wales had provided during other crises.

"I can't quite see the difference between beating back fires and beating back... floods and beating back this Covid-19 virus that could actually, if it gets worse here in New South Wales, could actually create massive problems for the whole country," Mr Hazzard told reporters.

The minister of health of Victoria state, Mr Martin Foley, said it would not send any of its vaccines to New South Wales, but would fully support the federal government sending any spare vaccines from a national stockpile to New South Wales, as it was clearly the state in greatest need.

Of the new cases in New South Wales, at least 45 of them spent time in the community while infectious, the state health authorities said. That figure is being closely watched as the state weighs whether to extend a lockdown due to end on July 30.

A particular problem was the spread of the virus through family visits, despite stay-at-home orders, Mr Hazzard said.

In one worrying example, 18 cases have been linked to a gathering after a family tragedy, New South Wales health director Jeremy McAnulty said.

Victoria, which includes Melbourne, reported 12 locally acquired cases of coronavirus on Saturday, down from 14 a day earlier, in what Foley called a reassuring sign as the second most populous state weighs whether to lift a hard lockdown from Tuesday night.

South Australia, also in lockdown, reported one new case, linked to a known outbreak.

Despite its struggle with spikes of infections, mostly of the Delta variant, Australia has managed to keep its epidemic largely under control with a total of about 32,600 cases and 916 deaths.