Authorities believe the man, who has not been vaccinated, has been infectious since Friday.
He had not been overseas or worked in Sydney's hotel quarantine system so authorities were keeping an open mind while working to find the source of the infection, NSW chief health officer Dr Kerry Chant said.
She said authorities hoped genome sequencing would identify the source of the infection, which could be from interstate.
Chant said the man's cycle threshold (VT) values were low, which meant he had a higher viral load and was potentially more infectious.
