Health officials were rushing to trace the source of a growing cluster of Covid-19 cases in Australia's largest city of Sydney, with thousands of residents urged to stay home.

Initial reports of five cases on Thursday morning had grown to 17 hours later, as officials worked to uncover the source of the infection in the city's northern beaches, AFP reported.

New South Wales Chief Health Officer Kerry Chant said at an earlier press conference that the cases were “a wake-up call” to Sydneysiders, who had been enjoying a relative return to normal in recent months.

“I think that it's fair to say that mask wearing has declined, with some level of complacency in the population,” Chant said. “And I think this is a wake-up call to urge the community to continue to follow that health advice.”