You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
Most businesses will close except for some food outlets, and people will be largely confined to their homes, as the state tries to avoid a more severe breakout like the one that all but shuttered neighbouring Victoria for more than 100 days.
"We need this circuit breaker, this community pause," South Australian Premier Steven Marshall told media on Wednesday.
"We are going hard and we are going early. Time is of the essence and we must act swiftly and decisively."
The southern state, home to just under 1.8 million people, reported two new cases on Wednesday, taking the total to 22. The cluster started to emerge at the weekend, representing the first serious signs of community transmission in South Australia since mid-April.
While the case numbers remain modest, Marshall said the state needed to act quickly because the identified virus strain was "highly contagious with short incubation period and limited symptoms".
Authorities said on Wednesday that a hotel cleaner contracted the virus from a quarantined returned traveller from the United Kingdom. The cleaner then spread the virus to her extended family in Adelaide, the state's capital city.
It was taking 24 hours or less for a newly infected person to become infectious to others, authorities said.
The South Australian government did not immediately respond to questions from Reuters about the specific strain on Wednesday.
Peter Collignon, an infectious diseases physician and microbiologist at Canberra Hospital, said given the virus was brought in from an overseas traveller, it would not be a new strain.
"It may be different to what has been prevalent in Australia but it isn't any more deadly or contagious," said Collignon.
The lockdown, to take effect from midnight Wednesday, will mean people will be restricted from going outside of their homes, with only one person per household allowed to leave each day, but only for specific purposes.
Masks will be required in all areas outside of the home.
All schools, takeaway food, pubs, cafes and universities will be closed, along with the construction industry, which has been allowed to operate during past lockdowns in Australia.
Australia's Acting Chief Medical Officer Paul Kelly said South Australian authorities had found secondary cases of COVID-19 were spreading quickly in the new outbreak.
"There are a range of options as to why that may be the case but at the moment we don't have any evidence that the virus has changed in any way to become more infectious or more dangerous," he said.
Beach Energy, Santos, BHP and Whyalla Steel are among resource companies with operations in South Australia. Beach Energy said it will not be hit by the lockdown while it was not clear whether others will be impacted.
Elsewhere, Victoria state, which was the epicentre of Australia's nearly 28,000 cases until last month, clocked its 19th straight day of zero new cases.
New South Wales state, of which Sydney is the capital city, reported zero local and seven imported cases.