Lockdown in Melbourne was due to end on Thursday, but authorities say this is no longer possible due to rising cases.

Australian authorities say they will extend a lockdown of the country’s second-biggest city, Melbourne, as they struggle to quell a coronavirus outbreak driven by the highly infectious Delta variant.

The announcement on Sunday came as the number of COVID-19 infections in Victoria state, of which Melbourne is the capital, rose by 92 overnight – the highest number in nearly a year.

There are also 778 active cases in Victoria, according to health data.

Melbourne’s lockdown, its sixth since the start of the pandemic, was due to end on Thursday. But Victoria Premier Dan Andrews said on Sunday that this would no longer be possible.

“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.

Melbourne’s lockdown is currently in its fourth week and includes a curfew, the closure of playgrounds and strict limits on exercise.

Hundreds of protesters march in an anti-lockdown protest in Melbourne, on August 21 [James Ross/AAP Image via AP]

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

Meanwhile, neighbouring 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 caseload to a new all-time high.

Nearly 19,000 cases have been detected in the state of about eight 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.

In non-hotspot areas, five fully vaccinated adults will be able to gather outdoors for up to an hour from mid-September while authorities have also signalled small weddings will soon be allowed.

Australian leaders have agreed on a national plan for reopening the country once vaccination targets of 70 and 80 percent are reached in each state and territory.

At present, only 33.7 percent of people older than 16 have been fully vaccinated. But at current rates, 80 percent could be vaccinated by mid-November.

“Learning to live with the virus is our only hope,” the daily The Age cited Australia Treasurer Josh Frydenberg as saying on Sunday. “To delay and deny that fact is not only wrong but incredibly unrealistic.”

Since the start of the pandemic, Australia has recorded more than 51,000 cases of COVID-19 and nearly 1,000 deaths in a population of 25 million.