South Australia's six-day lockdown is a "pre-emptive and temporary" measure to keep people safe and stay ahead of coronavirus outbreak, Prime Minister Scott Morrison says.
The lockdown will take effect from midnight on Wednesday, shutting down most businesses and services.
People will be asked to stay at home and only leave to buy groceries or seek medical treatment.
Flying back from Japan, Mr Morrison said he had spoken with Premier Steven Marshall and had remained in contact with Health Minister Greg Hunt and Acting Chief Medical Officer Paul Kelly.
"The establishment of a pre-emptive and temporary six-day lockdown to keep South Australians safe and stay ahead of the outbreak draws on lessons from earlier outbreak experiences," the prime minister said in a statement.
These are precautionary and temporary measures with a clear end date."
Mr Morrison said the federal government would continue to support SA in every way possible.
The Commonwealth has already approved the deployment of more ADF personnel to help with staffing at coronavirus testing stations.
The so-called Parafield cluster in Adelaide's north grew to 23 confirmed cases on Wednesday, however, seven more people are thought likely to be infected but are waiting on tests.
The outbreak was sparked by a woman who worked as a cleaner in the Peppers Hotel, one of Adelaide's quarantine facilities, who may have picked up the virus from a surface and then infected other family members
Genetic testing has linked her case back to a traveller who returned from the United Kingdom on November 2.