A public health emergency has been declared in South Australia because of deepening concerns about the spread of coronavirus.

Key points: SA Premier Steven Marshall said Australia was now in "uncharted waters"

SA Premier Steven Marshall said Australia was now in "uncharted waters" He said a public health emergency would ensure consistency with other states

He said a public health emergency would ensure consistency with other states Another person has tested positive to coronavirus in SA in the last 24 hours

The number of cases in the state has climbed to 20 — the latest being a woman aged in her 60s who recently returned from the US.

Premier Steven Marshall said public health officers had been empowered "to take all the necessary actions to keep the people of South Australia as safe as we possibly can".

"We have declared a public health emergency," the Premier said.

"These are uncharted waters for our nation."

It is understood to be the first time such a step has been taken in South Australia.

All of the state's cases have been people who had travelled overseas or interstate, or contracted the virus from a relative who had been overseas, Mr Marshall said.

He said there was "still no evidence" COVID-19 was spreading within the state.

"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," he said.

"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.

"This is a very difficult time … we're taking the threat of the coronavirus very, very seriously."

Mr Marshall today took part in the first meeting of the new national cabinet in response to coronavirus with other state leaders and the Prime Minister.

After announcing more stringent controls around people arriving in Australia, Scott Morrison said Queensland had already declared a coronavirus emergency, and left it up to the states to follow suit.

"They will be seeking to align how they do that over the next few days and that's a proactive thing to ensure we are getting on a consistent footing," he said.

SA Premier Steven Marshall supported tighter restrictions on people arriving or returning. ( ABC News: Stacey Lee )

The SA Premier said public compliance with guidelines had so far been good, but steps could be taken where breaches occurred.

"Authorities in South Australia do have the ability to take further action if they think there are people who are ignoring the directives," Mr Marshall said.

"We've got the ability under the Public Health Act already in place to not only direct people, but enforce that using SAPOL (SA Police)."

Latest case just got back from US

The number of coronavirus cases in South Australia has more than doubled in the last few days, with the total rising from seven a week ago.

The latest case was diagnosed after being swabbed by her GP upon returning from the US.

"She wasn't infectious on her flight, she was not showing any of the symptoms," Mr Marshall said.

Loading

The state's Communicable Diseases Control Branch is currently working to contact passengers who were on the flight.

However, the Premier was keen to reinforce that six of the state's 20 cases had since been discharged from hospital.

"Of those 20, 18 of them are people who were returning from overseas travel, one from interstate travel and one person who acquired this disease from living with a person who had contracted the coronavirus," he said.

"There is still no evidence whatsoever of community transmission in South Australia although this isn't the case in other jurisdictions."

Adelaide's Sacred Heart College will be closed on Monday after a Year 11 student tested positive for coronavirus on Thursday.

School authorities said the step was to "enable a deep clean" and ensure appropriate mitigation strategies were put in place.

The boy contracted the virus from one of his parents, despite not travelling overseas himself.

Mr Marshall supported the decision not to implement mass closures of schools, saying preliminary advice suggested such a step would be "counterproductive" and could "increase the risk".

The SA Opposition has urged the Government to provide free flu vaccines in a bid to reduce pressure on the healthcare system from the coronavirus pandemic.

The Government would not be drawn on that proposal, instead saying it was confident there were "necessary precautions" surrounding the flu vaccine.