Investigations are continuing into a COVID-19 case in South Australia to determine if the infection in the aged care worker, who recently returned from Victoria, will derail plans to ease border rules.
The woman in her 20s flew into Adelaide on Monday on a Jetstar flight and is in hotel quarantine.
Chief Public Health Officer Nicola Spurrier said the woman previously tested positive for coronavirus in Victoria in August but had been cleared.
Further tests are underway to determine if her case is an old infection and she is still shedding deadly virus, or if it is a rare case of reinfection.
Out of an abundance of caution the case was being considered infectious, Professor Spurrier said.
Premier Steven Marshall said SA would be guided by health advice as to whether the case would delay plans to lift the harder border closure with Victoria.
The state's transition committee is due to meet on Friday and is expected to consider relaxing the rules to allow people to travel into South Australia provided they home quarantine for 14 days.
That could clear the way for all border restrictions with Victoria to be lifted before Christmas.
Mr Marshall said the Christmas timetable remained a reasonable expectation, but the state would not ease restrictions until it was safe to do so.
"We need to see if this case sets us back at all," the premier said on Thursday.
"It is a complex case and we're trying to trace the origin."
South Australia reported one new virus case on Thursday, with a man testing positive after recently returning from overseas.
He has been in hotel quarantine since his arrival and poses no threat to the wider community.
There have now been 520 coronavirus cases in SA since the start of the pandemic but only 17 are still considered active.
They are all people in hotel isolation.