Sydney Airport has been declared a no-go zone for people travelling back to Adelaide amid the continued spread of COVID-19 in NSW.
Effective immediately, anyone arriving in Adelaide who has transited through Sydney will be required to spend two weeks in home quarantine.
The new rules are expected to have the most impact on people returning from regional Queensland, who are currently not required to isolate when they arrive back in SA.
But they also have the potential to affect people returning from other parts of the country who use a convoluted route that takes them through Sydney.
Police Commissioner Grant Stevens said with almost 200 virus cases reported in NSW on Tuesday, SA authorities no longer had confidence people travelling through Sydney Airport could "maintain a sterile corridor".
He's also reaffirmed warnings to people in Queensland to come home quickly, as the situation could change in the coming days, preventing any travel to SA except by special exemption.
"Anybody in regional Queensland needs to revisit their travel plans," Mr Stevens said.
"People should make every effort to return home as quickly as possible.
"We've given advance notice to these people. We're hopeful that people have heeded that message."
In respect to Brisbane and southeast Queensland, SA residents can still return but are already required to quarantine for 14 days.
SA also has hard borders in place with both NSW and Victoria.
The commissioner said SA remained on track to ease some local COVID-19 restrictions from Thursday, including relaxing the general density requirement in most venues to one person to every two square metres.
Sports competition can resume with spectator numbers at outdoor venues limited to 1000, but SA will keep mask-wearing in place for high-risk settings, high schools and most public places, including shopping centres.
Family gatherings will continue to be limited to 10 people, and weddings and funerals will stay at 50 people.
The changes come after SA stared down its own cluster of COVID-19 infections, sparked two weeks ago after a man returned from Argentina and tested positive after quarantining in Sydney.
The cluster grew to 21 cases and prompted a week-long statewide lockdown with authorities hailing the "go hard and go early" strategy for bringing it quickly under control.
South Australia reported no new coronavirus cases on Tuesday and currently has 24 active infections, all in quarantine.
Two people are in hospital, a man in his 80s and a woman in her 80s, but both are in a stable condition.
More than 10,000 tests were processed in SA on Monday, with the state now closing in on two million tests since the start of the pandemic.
It has also administered almost 900,000 vaccine doses.
Australian Associated Press