SYDNEY — Australia is on the verge of a national coronavirus outbreak just as most other developed economies are emerging from restrictions, with the delta variant seeding new virus clusters across the continent.

The highly transmissible variant, first detected in India, is confounding authorities’ ability to stay ahead of its spread, even with rigorous contact-tracing and domestic border closures that have allowed Australia to remain largely open for most of the pandemic.

Officials said it is a “critical time” for the nation. “We’re entering a new phase of this pandemic, with the more contagious delta strain,” Josh Frydenberg, the country’s top finance official, said in an interview aired on the Australian Broadcasting Corp.

A physiotherapist caught the virus in Sydney, probably through a fleeting interaction at a cafe, and flew 2,000 miles west to Perth. A flight attendant traveled up and down the country while potentially contagious. A gold miner in central Australia caught the virus and triggered a cluster spanning three states.

By Monday, the country’s most populous city, Sydney, and its smallest capital, Darwin, in the remote Northern Territory, were in lockdown. Mask mandates were introduced in the federal capital, Canberra; in tropical Queensland; and in Perth, capital of Western Australia state. Victoria, home to Melbourne, imposed restrictions along its borders with several states.

Hindering officials’ efforts to contain the virus’s spread: a sense of complacency that has crept into many communities after going months with effectively zero transmission of the virus.

Police issued fines to dozens of people over the weekend, including a Sydney family that ignored a lockdown order and drove to a horse-riding meet north of the city. Two naked sunbathers were fined for breaching coronavirus restrictions after they were startled by a deer and ran into a forest, where they became lost and had to be rescued.

About 5 million Sydney residents are subject to stay-at-home orders until at least July 9, after authorities on Saturday expanded a local lockdown. Residents are allowed to leave home only for essential work, outdoor exercise in groups of fewer than 10, medical care and groceries and other necessary services.

An outbreak in the city that began with an airport limousine driver has grown to 124 cases, including 15 new cases Monday. The driver was unvaccinated, authorities said.

“We rely on 100 percent of people doing the right thing 100 percent of the time,” Gladys Berejiklian, the premier of New South Wales state, told reporters on Monday.

A slow vaccination rate is exacerbating the struggle, with less than 5 percent of Australians fully vaccinated against the coronavirus. Health authorities on Monday cited the recent example of a super-spreading birthday party in Sydney, where nearly all of the 30 people who attended contracted the virus from a single guest. The handful who didn’t were vaccinated.

Australia is regularly hailed as one of the best places to be during the coronavirus pandemic, with low case numbers, comparatively few deaths and a strict quarantine system for returning citizens and residents. Medical experts hope the latest outbreaks will shake off some of the resulting hubris.

“We’ve been the victims of our own success so far because, to some degree, there’s been a level of complacency and we’ve been living in a very gilded cage, a ‘Truman Show’ … world where we’ve really been very disconnected,” Chris Moy, vice president of the Australian Medical Association, said Sunday. He was referring to the 1998 film about a man who lives in a picture-postcard town called Seahaven that is a little too perfect and is actually the huge set of a television show.

On Sunday evening, nearly 60,000 people attended an interstate rugby league clash in Brisbane, the capital of Queensland state. Most of the spectators were maskless.

Early the next morning, local health authorities announced the new mask order after a worker from the gold mine in central Australia tested positive upon entering the state — one of about 900 workers from the mine who had flown interstate across Australia.

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