A lockdown has been imposed on Perth after a quarantine hotel security guard tested positive for Covid.
It means Perth’s two million residents are under effective lockdown until 5 February.
They must stay home, except for essential work, grocery shopping or for medical reasons, said Western Australia state Premier Mark McGowan.
Perth had not recorded any new cases for 10 months until the weekend, and there had been no new cases across the country for two weeks.
Restaurants, cafes and cinemas were ordered to close and school holidays have been extended for a week.
McGowan defended the snap decision to go into lockdown with just one positive case.
"I know for many Western Australians this is going to come as a shock. We cannot forget how quickly this virus can spread, nor the devastation it can cause."
Residents in regional WA have been asked not to travel into Perth and police checkpoints are being set up.
Australia reopened its travel bubble with New Zealand on Sunday after suspending it for six days.
The federal government is beefing up precautions with all passengers coming from New Zealand being screened before and after inbound flights for at least the next 10 days.
Written by Ray Montgomery, Asia Editor