SA will keep its hard border closure with Victoria in place indefinitely after 428 new cases were reported there on Friday.

"There was a lot of talk about a second wave a month or two months ago and it was all very theoretical," Mr Marshall said on Friday.

"Now it has become a stark reality for our country with what's happening in Victoria.

"The problem with Victoria at the moment is their numbers are actually increasing, not decreasing.

"So we're going to be very cautious about lifting the border at this time. We do not want to go backwards."

Mr Marshall said South Australia would also keep its border quarantine measures in place with NSW and the ACT for the time being with concerns remaining over the cluster of cases linked to the Crossroads Hotel in Sydney's southwest.

Travellers from those regions must self isolate for 14 days.

On Thursday, SA reported one new case of coronavirus with a woman who recently returned from Afghanistan testing positive.

Health officials said the woman, aged in her 40s, was no longer infectious and was not considered a risk to the wider community.

She arrived in Adelaide on Sunday after spending two weeks in quarantine in Melbourne where she returned two negative tests to COVID-19.

But when she was tested after flying into SA, the result was a low positive, an indication she was in the recovery phase of the disease.

An antibody test showed the infection had probably occurred some weeks earlier.

Her case took SA's total since the start of the pandemic to 444.

No new cases were reported in South Australia on Friday.

From midnight on Saturday all travellers into SA from Victoria, mostly locals returning, must get tested for coronavirus within 24 hours or face a $1000 fine.

Australian Associated Press