MELBOURNE: More than five million people in Australia's second-biggest city of Melbourne were ordered into a snap week-long lockdown Thursday, as officials blamed a sluggish vaccine rollout and hotel quarantine failures for another virus outbreak.

Stay-at-home orders will apply to Melbourne and surrounding Victoria state from midnight for seven days, acting state Premier James Merlino told residents, as the cluster doubled to 26 cases.

"In the last day, we've seen more evidence we're dealing with a highly infectious strain of the virus, a variant of concern, which is running faster than we have ever recorded," Merlino said.

The variant detected is known as B1617 -- which has spread widely in India -- and is believed to have been transmitted from a traveller who returned to Australia from overseas.

Schools, pubs and restaurants are set to close, while gatherings will be banned and mask-wearing made mandatory.

New Zealand has already suspended its travel bubble with Victoria and while residents are now being shut out of other Australian states, prompting major airlines to slash Melbourne services.

The "circuit-breaker" lockdown is designed to allow contact tracers to get on top of cases, with residents permitted to leave home only for essential reasons including getting vaccinated.

Merlino said the conservative federal government's sluggish vaccine rollout was partly to blame for the latest lockdown, saying it was "not where it should be".

"If more people were vaccinated, we might be facing a very different set of circumstances than we are today. But sadly we are not," he said.

It is the fourth time Australia's second-biggest city has been plunged into lockdown since the pandemic began, including nearly four months of harsh restrictions last year.

Thousands of people across Melbourne were already self-isolating after positive cases attended dozens of locations including two separate Aussie Rules football matches, a series of nightclubs and a medieval battle re-enactment.