A woman in her 30s who recently returned from South Africa has tested positive to coronavirus while quarantining in Adelaide.
However, the NSW woman's case is not considered active and believed to be an old infection.
Deputy chief public health officer Mike Cusack said it was thought the woman had contracted COVID-19 while overseas.
He said she was currently displaying no symptoms and was not considered any risk to the wider community.
"Because you can continue to shed virus for some time after you've had an active infection, the presence of virus on those tests does not mean you're acutely infectious," Dr Cusack said.
"But this is her first COVID test and as it's the first time she's returned a positive result she's added to the South Australian tally."
He said the woman's antibodies were being checked to confirm it as an old infection.
Her case takes the SA total since the start of the pandemic to 465, but there are no active infections.
Testing across the state has climbed above 400,000 or equal to about 15 per cent of the population.
Meanwhile, SA has left its border restrictions with NSW and the ACT in place, requiring people arriving from those regions to quarantine for 14 days.
Dr Cusack said health officials were "heartened and encouraged" by the progress in NSW and would be looking at issues like community transmission over the coming days.
Premier Steven Marshall said while the number of cases in NSW were coming down the headline figure was not critical in terms of lifting the existing quarantine requirements.
He said local health officials were more interested in where those cases had occurred and in what circumstances.
"The top-line number, while interesting, doesn't really give a picture of the risk for South Australia," he said on Monday.
The premier said he did not expect any changes to the border rules this week.