The state of South Australia announced a six-day "circuit-breaker" lockdown for its nearly two million people to contain a coronavirus cluster in its capital city that ended a months-long streak of no infections.
Schools, restaurants and factories were told to close at midnight, while stay-at-home orders were issued for residents across the state.
It came as two new cases were linked to a cluster that emerged from an Adelaide hotel used to quarantine travellers from overseas, taking the outbreak to 22 cases.
Weddings and funerals will be banned and mask-wearing in public made mandatory in the state, which had not recorded a significant outbreak since April.
"We are going hard and we are going early," state premier Steven Marshall said.
"Time is of the essence and we must act swiftly and decisively. We cannot wait to see how bad this becomes."
The approach stands in stark contrast to the United States, where some politicians are refusing to implement virus measures even as case numbers surge, or Europe, where lockdowns were introduced only after infections spiralled.
South Australians have been told to only leave their homes for essential work, to buy groceries or for health reasons.
The state is the first in Australia to ban outdoor exercise for all residents since the pandemic began.
Chief health officer Nicola Spurrier said the "extreme" measures would give the state of 1.8 million people time to get on top of contact tracing and halt chains of transmission.
"I cannot be making this decision in two or three weeks' time or even two or three days' time because it is going to be too late," she said.
On Monday, as 17 cases were confirmed, officials had begun ordering thousands of suspected close contacts to self-isolate, and suspended international flights.
Adelaide residents have since been flocking to Covid-19 testing sites, with many forced to wait several hours in long queues to be seen by overwhelmed clinicians.
South Australia's new restrictions come amid fears the latest outbreak has the potential to infect high-risk populations, with care workers and a prison guard among those testing positive.
Ms Spurrier said the state hoped to avoid a lengthy Melbourne-style lockdown, where residents in Australia's second-largest city spent months mostly confined to their homes after security bungles at a hotel quarantine.
The announcement sparked a fresh round of panic-buying in Adelaide, despite assurances that supermarkets would remain open.
"We will have police officers on standby to attend if we see any civil disorder and we would take action. This is completely unacceptable," South Australia police commissioner Grant Stevens said.
Melbourne, which recorded thousands of cases and hundreds of deaths at the height of its outbreak, has begun easing restrictions after more than two weeks without any new cases.
Other regions, where the virus has largely been stamped out, have imposed new quarantine rules on anyone travelling from South Australia.
The country's internal borders had been gradually reopening and were due to be almost fully reopened by Christmas.
Australia has been relatively successful in containing the virus, with just over 27,700 cases and 907 deaths recorded since the pandemic began.