SYDNEY: Australia's prime minister announced 300,000 coronavirus vaccine doses will be rushed to Sydney Thursday, as the country's largest city struggled to bring a Delta outbreak under control.

As a citywide lockdown entered its third week, there were signs of the outbreak spiralling, with a record increase of 38 new cases in the last 24 hours.

Scott Morrison said the situation in the city was "very serious" and urged five million Sydneysiders not to give in to fatigue and obey stay-at-home orders.

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The outbreak has nearly reached 400 cases, and is spreading quickly across the largely unvaccinated city, putting Australia's Covid-zero status at serious risk.

"We have come so far over these last 18 months, and now's the time to keep pressing forward. Now is not the time to give in to that frustration," Morrison told reporters in Sydney.

Police announced they would be bolstering patrols in the city's southwest to enforce lockdown rules as case numbers rise.

Despite several outbreaks, Australia has managed to avoid the worst of the pandemic through snap lockdowns, intensive contact tracing, and effectively shutting its borders to the rest of the world.

After a top New South Wales health official suggested it might be impossible to bring the outbreak under control, regional leaders threatened to cut Sydney off from the rest of the country.

The premier of Western Australia vowed to indefinitely extend a ban on travellers from the city and surrounding New South Wales "if they don't get it under control".

"I think that's a statement of the bleeding obvious," Mark McGowan said.

Travel bans between states have become a regular occurrence in Australia, but the threat of closing borders until vaccines are rolled out was a marked escalation.

Around eight percent of Australians have been fully vaccinated -- one of the lowest rates among rich nations -- and widespread vaccination is not expected to be in place until the end of 2021.

Australian Medical Association president Omar Khorshid insisted "there was no alternative to elimination" and said tighter restrictions may be needed to stop community spread.

"Nowhere in the world has any community been able to live with Delta without very significant levels of vaccination, we are simply not ready to live with this Delta strain," Khorshid said.

Since the pandemic began, Australia has detected just over 30,800 virus cases and reported 910 deaths.