Australian officials extended lockdown and social distancing measures to more of the country on Wednesday, with four major cities already under a hard lockdown in a race to contain an outbreak of the highly contagious Delta coronavirus variant.

Around one in two Australians are under stay-at-home orders, with millions of others subjected to movement curbs and mandatory mask-wearing amid COVID-19 flare-ups in several locations.

With more than five million residents of greater Sydney under a two-week lockdown until July 9, New South Wales (NSW) state reported 22 new locally transmitted COVID-19 cases on Wednesday, all linked to prior infections.

That was up slightly from the previous two days, but still below the peak of the current outbreak of 30 new cases reported on Sunday.

"New South Wales is demonstrating a steady rate of cases at this stage, but to date our fears about huge escalation haven't materialized and we certainly want to keep it that way," state Premier Gladys Berejiklian said in Sydney.

With a total of around 170 new locally transmitted cases since the first infection was detected two weeks ago in a limousine driver who transported overseas airline crew, NSW is the worst-affected state or territory in the current outbreak.

Residents of Sydney, Perth, Brisbane and Darwin were joined in lockdown on Wednesday by those of the outback town of Alice Springs, the gateway to UNESCO World Heritage-listed Uluru. Officials issued stay-at-home orders for the town after a potentially infected traveler used the airport.

South Australia, meanwhile, reported its first locally transmitted cases for 2021, but stopped short of imposing a full lockdown, saying they believed the threat was contained.

Officials instead limited home gathering and urged people to wear masks in public after they reported five new cases – a miner who had returned home from a Northern Territory mine and his wife and children who had been in self-isolation.

Elsewhere in the country, Queensland reported three new locally acquired cases, Western Australia logged one and the Northern Territory recorded none.

Lockdowns, tough social distancing, swift contact tracing and a high community compliance have helped Australia quash prior outbreaks and keep its COVID-19 numbers relatively low. It has reported just over 30,550 cases and 910 deaths since the pandemic began.

But less than 5 percent of its 20 million adult population has been fully vaccinated, leading to criticism of a sluggish national inoculation drive.