South Australians are spending their first day in lockdown as the state embarks on a six-day "circuit breaker" to contain a COVID-19 cluster.
The government says the pause in most community activity will significantly reduce the risk of the virus spreading further.
It could also prevent the need for a much longer Victorian-style shutdown.
"We need this circuit breaker, this community pause. We are at a critical point," Premier Steven Marshall said.
From Thursday all schools are closed along with universities, pubs, cafes, retail stores, food courts and takeaway food outlets.
Regional travel is banned and aged care centres are in lockdown.
Factories are closed, along with the construction industry, and elective surgery has ceased.
Weddings and funerals are banned along with all outdoor sport and exercise and masks will be required outside the home.
People who are not essential workers will only be allowed to leave their homes once each day to buy groceries or to seek a COVID-19 test or other medical treatment.
Supermarkets, petrol stations, medical centres, critical infrastructure, public transport, airport and freight services, banks, post offices, school and childcare for essential workers and veterinary services are open.
Face masks must be worn outside the home.
The business sector has backed the lockdown but said it could be devastating for the state economy.
Prime Minister Scott Morrison described it as a "pre-emptive and temporary" strategy to keep South Australians safe.
"These are precautionary and temporary measures with a clear end date," he said.
All being well, the lockdown will be replaced by eight further days of heavy but less stringent restrictions.
The measures are the government's response to the so-called Parafield cluster, which increased to 23 confirmed cases on Wednesday.
Chief Public Health Officer Nicola Spurrier said it was vital to act quickly to stop the chains of transmission.
A number of sites across Adelaide are of key concern, particularly a pizza bar, where a worker tested positive, two northern suburbs schools, a hospital and a swimming centre.
South Australian Chief Public Health Officer Nicola Spurrier says it's vital to act quickly to curb Adelaide's COVID-19 cluster. Source: AAP
People who visited those sites have been asked to quarantine and get tested.
Almost 40 other locations are listed as places confirmed cases have visited in recent days, with people there at the same time asked to monitor for symptoms and get tested if they feel ill.
Other states caution against travel
Earlier, NSW Premier Gladys Berejiklian said residents of her state should delay travelling to South Australia "as a precaution".
The NSW premier said the state’s borders were "completely open" but stressed, "if it's not essential travel think about whether you want to go in the next few days".
"We are confident that South Australia has it under control but we're just saying to our community ... if you can delay it for a few days unless you have to go there please do so," she told reporters on Wednesday.
Victoria, meanwhile, has asked for South Australians to cancel all non-essential travel.
Victorian Premier Daniel Andrews warned he would shut the border if people ignored travel advice and crossed the border unnecessarily.
"If they do well in containing this outbreak, every Australian does well," he said.
Federal authorities are keeping a close eye on Adelaide's coronavirus cluster, with more Australian Defence Force troops preparing to travel to South Australia.
The state has asked for an additional 45 troops to join the 100 already on the ground.
Before the lockdown announcement, Deputy Prime Minister Michael McCormack said there was no reason for interstate travel restrictions.
"We do need Australians to get back to work and we do need those borders to be open," he told the Nine Network on Wednesday.
All hotel quarantine workers around Australia will now be tested weekly.
There are still more than 35,000 Australians stranded overseas trying to return home.
People in Australia must stay at least 1.5 metres away from others. Check your jurisdiction's restrictions on gathering limits.
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