A pre-Christmas coronavirus outbreak in Sydney is believed to have spread to the city of Melbourne, breaking its two month run of zero community infections.
Melbourne, Australia's second largest city, endured a heavily-policed lockdown from July to October which succeeded in eradicating a second wave.
But on Thursday, officials reported six new cases with a "potential link" to a traveller from New South Wales, of which Sydney is the state capital.
Authorities have vowed to act quickly.
A mandatory mask-wearing order and gathering limits have been reimposed on Melbourne by officials in the state of Victoria, who described the outbreak as a "very serious situation".
"Victorians who have sacrificed so much are not going to go back to a situation where we face what we did in the past winter," said state health minister Martin Foley.
