A coronavirus curfew was announced Monday for Australia's second-biggest city of Melbourne, with residents confined to their homes overnight as authorities work to stamp out a Delta variant outbreak.

More than five million Melbourne residents will be unable to leave their homes between 9:00 pm and 5:00 am from Monday evening, with essential workers requiring permits to be on the streets.

Victoria state premier Dan Andrews said the decision came after a series of street parties, pub crawls and home gatherings over the weekend, as Melbourne reached a "tipping point" in its latest outbreak.

"We've seen lots of different people flouting these rules, not doing as they should, making really poor choices," he said.

Andrews also announced that stay-at-home restrictions would be extended to September 2, saying it was necessary for the city to avoid the fate of Sydney "where it has fundamentally got away from them".

More than eight million people in New South Wales state are under lockdown -- including in Sydney where residents have already been under those restrictions for almost two months.

Australia's most populous state has recorded more than 8,200 cases since the outbreak began in mid-June, with 56 related deaths in a population with low vaccination rates.

A snap lockdown was announced for the northern city of Darwin on Monday, with about 150,000 people told to stop non-essential movements for three days after a man tested positive.

"We have made the decision to lock down fast because of what we do not know," Northern Territory chief minister Michael Gunner said.

"There is a very real risk that this virus has been transmitted to others."

Australia's capital Canberra will also remain in lockdown until September 2 as health officials grapple with a small but growing outbreak in the city.

Australia won global praise for its early pandemic response but strict border closures and other measures have struggled to contain the highly transmissible Delta variant.

Repeated and lengthy lockdowns in some cities have left many people weary of restrictions, as authorities pursue an increasingly elusive "Covid zero" status until vaccination rates reach 70 percent or higher.

After a glacial rollout, nationwide vaccination efforts have accelerated in recent weeks with a quarter of Australians now fully inoculated and supplies beginning to ramp up.

Australia has recorded more than 39,000 cases of Covid-19 and 966 deaths to date in a population of 25 million.