Nations from Australia to Vietnam announced more drastic curbs and longer lockdowns for citizens on Friday, as authorities struggle to rein in outbreaks of the highly infectious Delta variant of the coronavirus.

In Australia's biggest city of Sydney, 2 million residents, or roughly 40% of its population, face curfew from 9 p.m. to 5 a.m. next week, along with limits on exercise.

A lockdown of the entire city, now in its eighth week, was also extended until the end of September, with residents told to wear masks outdoors, except for exercise.

"I asked health and police to work together, to give me a final list of what we can throw at this, to leave no shadow of a doubt as to how serious we are," Gladys Berejiklian, the premier of Sydney's home state of New South Wales, told reporters.

The state, which is Australia's most populous, reported 644 new infections on Friday, while southeastern Victoria state, home to Melbourne, recorded 55, as authorities there warned of a risk the outbreak could slip out of control.

In neighbouring New Zealand, an outbreak that began this week widened to the capital, Wellington, from the biggest city of Auckland, prompting Prime Minister Jacinda Ardern to extend a nationwide lockdown until Tuesday.

Though Friday's 11 new infections are relatively few, Ardern's critics question if she can repeat last year's feat of almost stamping out COVID-19, given the highly contagious nature of the Delta variant.

Vietnam imposed its toughest order yet, barring residents of the business hub of Ho Chi Minh City from leaving their homes from Monday, although further details have yet to be announced.

"Each home, company, factory should be an anti-virus fort," said Pham Duc Hai, deputy head of the coronavirus authority in Vietnam's biggest city, adding that people were being asked not to go outdoors.

Like Australia and New Zealand, Vietnam was once lauded for success in containing the virus, but is finding earlier measures were insufficient.

It has more than 312,000 infections and 7,150 deaths, up from a May 1 figure of less than 3,000 cases and 35 deaths.

Japan plans to roughly triple daily COVID-19 tests to 320,000, an acknowledgement that its main containment strategy of breaking up clusters of infections was no longer working in big cities like Tokyo, the capital.

The move comes after new daily cases exceeded 25,000 on Thursday for the first time.

South Korea extended for two weeks social distancing curbs that include a ban on gatherings of more than two people after 6 p.m.

Thailand, which had also managed to hold down infection numbers until the Delta variant hit, said its cases had passed the 1-million mark on Friday.