A crew member aboard a ship known to have travelled from China was assessed for symptoms of coronavirus by the ambulance service off Whyalla on Monday.
South Australian Ambulance Service crews, who reached the ship by boat, determined that the patient did not fit the criteria for COVID-19.
The patient was transported to the Whyalla Hospital and investigated for a viral infection unrelated to coronavirus.
The Whyalla News understands the crew member was suffering from gastro-enteritis.
An ambulance spokesperson said the incident had been "treated with suspicion to ensure infection control and to protect the safety of our crews".
"We are well prepared for these scenarios and have robust procedures in place," the spokesperson said.
The Health and Wellbeing Department said COVID-19 was part of routine respiratory infection testing.
"We will report on the number of positive cases and cases specifically under investigation by the Communicable Disease Control Branch, rather than the total number of tests performed," it said.
It comes as tough new laws aimed at stalling the spread of coronavirus are being rushed through Parliament by the state government.
Premier Steven Marshall said the amendments will give health authorities the powers they need to protect South Australians.
Under the changes, patients can be ordered to remain at a hospital or quarantine facility or require them to undergo testing or counselling.
Opposition Leader Peter Malinauskas said Labor would support the new laws.