Perth, the capital of Western Australia state, began a four-day lockdown at midnight Tuesday, while Brisbane, the capital of Queensland, will enter a three-day lockdown starting Tuesday evening.
They join New South Wales’ capital, Sydney, and Darwin, capital of the Northern Territory, which are already under lockdown.
Australia was celebrated for its initial response to the pandemic and for getting its economy more or less back on track long ago.
Residents of Perth and the neighboring Peel region must stay home except for urgent reasons after officials detected a third case, linked to the outbreak in Sydney.
And in Brisbane, the country’s third largest city, people will be required to stay home except for essential work, health care, grocery shopping or exercise.
Sydney, home to a fifth of Australia’s 25 million population, is under a two-week lockdown until July 9, while the lockdown in Darwin was extended for another 72 hours until Friday.
Concerns the Delta strain could spark significant outbreaks have also seen some form of restrictions in most other cities, with more than 20 million Australians, or about 80% of the population affected.
Authorities late on Monday said people age under 60 would now be able to get the AstraZeneca Covid-19 vaccine if approved by their doctor, who will be covered by a no-fault indemnity scheme, to ramp up a sluggish immunization drive.
Vaccinations have also been made mandatory for high-risk aged-care workers and employees in quarantine hotels.
“I’m absolutely comfortable about the rollout through the aged care sector,” Australia’s chief medical officer Paul Kelly told the Australian Broadcasting Corporation on Tuesday.
Lockdowns, swift contact tracing and strict social distancing rules have largely helped Australia keep its Covid-19 numbers relatively low, with just over 30,500 cases and 910 deaths — but its vaccine rollout has hit several roadblocks.
Officials two weeks ago limited the use of AstraZeneca vaccines only to people over 60 due to blood clot concerns, while recommending Pfizer shots to everyone under 60 in a major change to its immunization drive.
CNN’s Angela Dewan contributed to this report.