Mr Marshall says by maintaining the strict border measures, SA has been able to accelerate the lifting of controls placed on businesses and the wider community.

"If we open up all the borders we can really only move as fast as the slowest jurisdiction," he told reporters on Thursday.

"On balance, we've formed the opinion that it's more important for us to get people back to work in South Australia.

"Therefore we've made the decision to keep our borders closed and go ahead with lifting restrictions and creating work."

The premier's comments came after Australia's Deputy Chief Medical Officer Paul Kelly said domestic border closures were never part of the national response and no medical reasons remained to keep them closed.

"At that time we were seeing large numbers of cases starting to develop, particularly in New South Wales and Victoria. And so I can understand why those decisions were made," he said.

"If we look at the situation now, several of those states and territories have not had any cases for the last few days."

"From a medical point of view I can't see why the borders are still closed but, as I said, that's for the states and territories themselves to decide when that time is right for them."

Mr Marshall said there would come a time to open the borders but only when other state's had similarly low or no new virus cases.

"That's not the position at the moment," he said.

"I fully appreciate that it's extraordinarily difficult for business and individuals that are affected by these closures.

"But we're doing it for good reasons, for health reasons but also for economic reasons."

SA currently has no active cases of COVID-19 and has reported only one new infection over the past month.

Australian Associated Press