NSW Premier Gladys Berejiklian says the man in his 50s has not recently travelled overseas and does not work in a hotel quarantine, border or a health role.
He was tested on Tuesday and the positive result was recorded on Wednesday.
NSW Health contract tracers are urgently working to identify any contacts.
Chief Health Officer Kerry Chant said the man is believed to have been infectious since last Friday.
Genome sequencing is underway, with results expected in the next 24 hours.
He has a number of close contacts who were being tested and were isolating, Dr Chant said.
He visited a number of venues while potentially infectious, she said.