People wearing masks alight from a tram in Melbourne on Wednesday, as Australia's second biggest city scrambles to contain a growing COVID-19 outbreak. The country has reported more than 30,000 infections, with about 910 people dying from the deadly disease. Photo: AFP

A lockdown of Australia's second-biggest city Melbourne will be extended, authorities announced Sunday as they struggle to quash a stubborn coronavirus Delta variant outbreak.

Almost 7 million people in Melbourne and surrounding Victoria state were scheduled to exit a four-week lockdown on Thursday, but state premier Dan Andrews said it would no longer be possible with case numbers rising by 92 overnight.

It is the city's sixth lockdown of the pandemic, and includes a curfew, the closure of playgrounds and strict limits on exercise.

"We still have too many cases in the community for too long for us to be able to open up and give back... those freedoms that we cherish and those freedoms that we desperately want back," Andrews said.

Andrews did not reveal how long stay-at-home orders would remain in place, saying officials would "look at all the different options."

Meanwhile, New South Wales state, which includes Australia's most populous city of Sydney, posted 1,218 new cases on Sunday - pushing the country's overall daily case load to a fresh all-time high.

Almost 19,000 cases have been detected in the state of about 8 million people since the Delta variant outbreak began in mid-June.

But with vaccination rates now surging in New South Wales and authorities predicting 70 percent of adults there will be fully vaccinated by October, residents weary of prolonged restrictions have been promised some modest freedoms.

AFP