The issue is expected to be discussed at a national cabinet meeting on Thursday but Mr Marshall said SA would not be adopting such a change.
"That's not one that we will be signing up to," the premier told ABC radio.
"We know that patients still have symptoms after five days, in fact after seven days.
"We know that from the surveys that we take. We don't want a whole pile of people who are infectious post contracting this disease out getting more people infected.
Mr Marshall said surveys and checks on people with COVID-19 had confirmed a large number of people still had symptoms after day five.
"We would just be literally lighting a fire in South Australia if we allow them to leave quarantine after five days," he said.
"They would just be out there infecting other people and then we'd have another wave in South Australia."
The premier said a shift to a seven-day quarantine period was possible at some stage, but at the moment he believed SA had the balance right.
"We do have a more prudent approach in South Australia but I think over the longer term that's proved to be the most successful," he said.
Mr Marshall's comments came after SA reported 3482 new infections on Wednesday, below the state's seven-day average of 3933.
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