Australians have been warned they face the most “serious crisis” in the COVID pandemic since last February/March as health officials battle to contain new outbreaks of the virus.

Australia’s COVID-19 committee is due to hold an emergency meeting on Monday over rising case numbers plus outbreaks of the Delta variant across the country.

Authorities in New South Wales are warning coronavirus infections will increase “considerably” after the state recorded 18 new locally transmitted virus cases.

A two-week lockdown has been imposed in the state’s capital Sydney after a cluster of cases of the Delta variant rose to triple figures.

“We have to be prepared for the numbers to bounce around and we also have to be prepared for the numbers to go up considerably,” New South Wales Premier Gladys Berejiklian said.

Health policy adviser Bill Bowtell warned: “We really face the most serious crisis in the COVID pandemic since the early days in February-March last year.”

The northern city of Darwin has entered a two-day shutdown after a gold mine worker tested positive for the variant, first identified in India.

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They are suspected to have become infected while in hotel quarantine in Brisbane in Queensland state before flying to a gold mine in the Northern Territory.

Authorities are now trying to track down 900 mine workers around the country who could have been infected by the initial case.

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Around 18 million Australians, or around 70% of the population, are now under some form of lockdown or restrictions as officials grapple with increasing COVID-19 infections in almost every state or territory.

“I think we are entering a new phase of this pandemic, with the more contagious Delta strain,” federal treasurer Josh Frydenberg told the Australian Broadcasting Corp.

He said the country is facing a “critical time” in its fight against COVID-19.

Mandatory face masks have been reintroduced in Queensland and curbs on home gatherings imposed in areas including the state capital Brisbane.

It follows similar action by Western Australia officials for state capital Perth, after a resident tested positive following a trip to Sydney more than a week ago.

Restrictions also remain in the country’s capital Canberra, as well as Victoria state capital Melbourne.

South Australia state has announced new state-wide restrictions from Tuesday such as mask-wearing, limits on private gatherings – including weddings and funerals – and new social distancing rules in pubs and clubs.

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Australia has so far been relatively more successful than many other developed countries in containing the spread of the coronavirus throughout the pandemic, with just over 30,450 cases and 910 deaths.

Lockdowns, tough social distancing rules and swift contact tracing have helped the country suppress previous outbreaks, but the fast moving Delta variant has alarmed health authorities.

It has prompted fresh calls from state officials on the federal government to tighten already tough border restrictions to reduce the number of travellers arriving in the country.

The new clusters have also highlighted the nation’s slow vaccine rollout with only 5% of the population fully vaccinated.