SYDNEY (BLOOMBERG, REUTERS) - The Australian city of Darwin joined Sydney in lockdown as the nation grapples with a widening Covid-19 outbreak that includes the highly contagious Delta strain.

Darwin, the capital of the Northern Territory, was put into a snap, 48-hour lockdown starting on Sunday (June 27) after four new local cases were detected.

Earlier, officials in the state of New South Wales warned that they expect the number of infections in Sydney's Delta-variant outbreak to increase over the next few days.

"I cannot rule out the lockdown being longer," the Northern Territory's Chief Minister Michael Gunner said in a briefing. The lockdown decision was made reluctantly, "but we know hard and early is better than late", he said.

Darwin's lockdown is linked to a Newmont gold mine worker in the Tanami Desert in the central Northern Territory. Nine hundred people who left the mine between the infectious period from June 18 to June 25 were still in the Territory, the local government said in a press release.

New South Wales recorded 30 new local cases in the 24 hours through Saturday night, state Premier Gladys Berejiklian said in an earlier briefing. Some of those people were in the community while potentially infectious, she said.

Greater Sydney is locked down until July 9 to quell a flareup that began in mid-June. The number of infections linked to the Bondi outbreak stood at 110 on Sunday, and two other cases remain under investigation. Some 52,000 tests were conducted.

There are concerns over the risk of outbreaks in other parts of the nation.

Contact tracers are striving to keep up with more exposure sites, including some domestic Virgin Australia flights after a cabin crew member tested positive.

"Given how contagious this strain of the virus is, we do anticipate that in the next few days case numbers are likely to increase even beyond what we have seen today," Ms Berejiklian said.

Meanwhile, the state of Queensland announced two new local cases and imposed additional curbs to help combat the virus.

Australia is under pressure to step up vaccinations as the nation's roll-out trails that of the United States. Britain and parts of Europe, which are opening up their economies.