SYDNEY—More parts of Australia have been locked down as authorities try to locate workers from a remote gold mine linked to five Covid-19 cases thought to be of the highly infectious Delta strain.

The far north city of Darwin and two nearby areas were locked down on Sunday for 48 hours after officials failed to locate 15 of 211 workers who flew into Darwin from the Granites Mine in central Australia. The Northern Territory, which covers 548,000 square miles of mostly desert, reported four new Covid-19 cases on Sunday, all linked to a worker previously identified as positive.

“The safest thing to do is to assume the worst, assume they are positive and assume there are exposure sites,” Northern Territory Chief Minister Michael Gunner said.

Mr. Gunner said all cases at the mine, which is about 330 miles north of iconic Uluru, were highly likely to be of the Delta variant. He said the outbreak represented the biggest threat to his region since the pandemic began.

The first Covid-19 case from the mine was diagnosed on Saturday. About 750 workers are in isolation on site, while about 900 workers have dispersed across the territory and into neighboring Queensland and Western Australia states.

The Sydney outbreak that has forced Australia’s most populous city and surrounding areas into a two-week lockdown is likely to yield increasing numbers of cases over the next few days, the New South Wales state premier said. The state reported 30 new locally acquired cases on Sunday.

A Covid-19 vaccination center in the west of Sydney has been identified as a potential exposure site, while authorities have ordered passengers and crew from five domestic flights to isolate after the Virgin Australia airline said a cabin crew member had tested positive and had been infectious while working.

About 75% of New South Wales state’s 8.2 million people have been in lockdown since Saturday afternoon. The outbreaks in Australia prompted New Zealand to suspend the countries’ so-called travel bubble, which has been operating since April, over the weekend.

Helped by its island status and regular restrictions, Australia has reported fewer than 31,000 Covid-19 cases since the start of the pandemic, mostly among travelers returning from overseas. Less than 5% of the 26 million population is fully vaccinated, and state governments have criticized the federal government over levels of vaccine supply.