The man in his 20s was a close contact of a teenage boy -- a member of the Jets' under-15s squad -- who had tested positive for COVID-19.
Hunter New England Health urged any patrons at Sunday's game at McDonald Jones Stadium to "be alert for symptoms, and if any develop to get tested and self-isolate immediately."
There were 2,570 spectators at Sunday's game, which the Jets won 1-0, but only the lower level of McDonald Jones Stadium was open, with seating in the bays capped at 25 percent capacity to allow for social distancing.
The teenage boy who tested positive had also played a soccer match for Newcastle's under-15s against Stanmore Hawks on Aug. 1.
His teammates and the opposition were both considered close contacts and required to isolate for 14 days.
The FFA, Newcastle Jets and Western United have been contacted for comment.
The A-League season is being completed in NSW, with interstate clubs based in hubs.
Melbourne City and Brisbane Roar are both based in the Hunter Valley.
Newcastle previously had a A-League squad member test positive after their match against City in March, which was the final game before the season was suspended.