A public health emergency has been declared in SA, as the state records its 20th case of coronavirus.
A woman aged in her 60s was diagnosed with COVID-19 after recently returning from the United States, Premier Steven Marshall told reporters on Sunday.
"She wasn't infectious on her flight, she was not showing any of the symptoms," Mr Marshall said, noting authorities were working to contact passengers who were on the flight.
The woman was diagnosed after being tested by her GP after returning from the US.
"This is perfectly consistent with the other cases that have been identified here in South Australia," Mr Marshall said.
The premier said all of the state's cases have been people who had travelled overseas, interstate or contracted the virus from a relative who had been overseas.
"There is still no evidence whatsoever of community transmission in South Australia," he said.
"We're at a much lower level of risk than other jurisdictions but I have made the decision that we will work together with the other states."
As Australia is now in "uncharted waters", Mr Marshall said it was necessary to declare a public health emergency to both "keep the people of South Australia as safe as we possibly can" and ensure consistency with other states.
"We don't want people in South Australia being confused about the type of restrictions that might exist in New South Wales, Victoria and Queensland versus what happens here," he said.
The declaration was made under Public Health Act powers and it is understood to be the first time such a step has been taken in South Australia.
"This is a very difficult time," the premier said.
"We're taking the threat of the coronavirus very seriously."
Mr Marshall took part in the first meeting of the new national cabinet in response to coronavirus with other state leaders and Prime Minister Scott Morrison on Sunday.
He said he supported the prime minister's decision not to force mass closures of schools, suggesting such a step would be "counterproductive" and could "increase the risk" of students contracting the virus.
"Individual schools can make their own decisions and in fact, parents can make their own decisions but as a state, we have to rely on the best expert advice," he said.