Five million residents of the Australian city of Melbourne have been told to stay at home for six weeks, amid a surge in coronavirus cases.
Victoria State Premier Daniel Andrews announced the lockdown after the state saw 191 new infections, its highest daily number since the pandemic began.
The state’s borders with neighbours New South Wales and South Australia closed at midnight local time (14:00 GMT).
Australia has recorded almost 9,000 cases and 106 deaths from the virus. But almost all new cases have been in Victoria.
“We are on the cusp of something very, very bad if we don’t take these steps,” Mr Andrews said.
“I think a sense of complacency has crept into us as we let our frustrations get the better of us.” The state had been easing restrictions before the surge in the past fortnight.
It now has hundreds of active cases, while other Australian states and territories continue to report only small numbers of new infections.
People will be kept to their homes and will only be able to leave for essential reasons, such as for work, exercise and shopping for food and other necessities.
Schools will largely return to distance learning and restaurants will, once again, only be permitted to serve takeaway food.
However, shops and hairdressers will remain open. The measures were to come into force from Wednesday.
The lockdown covers only Melbourne and an area to the north called the Mitchell shire, but the state as a whole has been sealed off from the neighbouring states of New South Wales and South Australia.
Police and troops will be stationed at border crossings and patrol the vast borders with drones and other aircraft.