CANBERRA, Australia — Australia was battling to contain several COVID-19 clusters around the country on Monday in what some experts have described as the nation's most dangerous stage of the pandemic since the earliest days.

Sydney in the east and Darwin in the north were locked down on Monday. Perth in the west made masks compulsory for three days and warned a lockdown could follow after a resident tested positive after visiting Sydney more than a week ago.

Brisbane and Canberra have or will soon make wearing masks compulsory. South Australia state announced new statewide restrictions from Tuesday. 

Australia has been relatively successful in containing clusters throughout the pandemic, registering fewer than 31,000 cases since the pandemic began. But the new clusters have highlighted the nation's slow vaccine rollout with only 5% of the population fully vaccinated.

Most of the new cases stem from a Sydney limousine driver who tested positive on June 16 to the delta variant, which is thought to be more contagious. He was not vaccinated, reportedly did not wear a mask and is suspected to have been infected while transporting a foreign air crew from Sydney Airport.

New South Wales state on Monday reported 18 new cases in the latest 24-hour period. The tally was fewer than 30 cases recorded on Sunday and 29 on Saturday.

Authorities warned that a two-week Sydney lockdown that began on Friday would not reduce infection rates for another five days.

"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, who was the architect of Australia's first AIDS response in the 1980s, said the government needed to consider hastening vaccinations by shortening the gap between AstraZeneca shots from 12 to 8 weeks.

"We really face the most serious crisis in the COVID pandemic since the early days in February-March last year," Bowtell said.

The crisis has also highlighted the dangers posed by hotel quarantine, which is the source of most cases of community virus spread in Australia.

A mine worker is suspected to have become infected with the delta variant while in hotel quarantine in Brisbane in Queensland state before flying to a gold mine in the Northern Territory.

The miner infected at least six people at the mine. One of the infected miners had since traveled home to Queensland and another to New South Wales.

Authorities were attempting to track down 900 mine workers around the country who could have been infected by the initial case.

The Northern Territory capital Darwin, and neighboring Palmerston, on Sunday locked down for 48 hours after an infected miner returned home to Palmerston.

That lockdown would be extended to Friday after another miner tested positive after returning home to Darwin on Friday, officials said on Monday. The Northern Territory has never before experienced COVID-19 spreading in the community. 

Queensland on Monday reported three new cases, including the miner. She is one of 170 potentially infected miners who live in the state and fly to an from work.

Masks will become compulsory from Tuesday for two weeks in Brisbane and several surrounding towns.

"The next 24-to-48 hours are going to be very crucial in Queensland about whether or not we see any spread of this delta strain," Queensland Premier Annastacia Palaszczu said.

The Queensland government has called on the federal government to tighten already tough border restrictions to reduce the number of travelers arriving in Australia.

Western Australia state reported one new case in Perth linked to the Sydney cluster. The state is home to 177 of the potentially infected miners.