Souths chief executive Shane Richardson made a formal complaint to the stadium manager and Panthers general football manager Phil Moss.
The NRL's integrity unit are expected to investigate the incident.
Panthers officials confirmed the club were made aware of the allegations yesterday and will be investigating.
The club said a female spectator had contacted the club regarding the incident and has agreed to co-operate.
The upsets continued in this year's NRL, with the Eels beating defending NRL premiers Sydney Roosters 14-12 last night.
The Knights also beat the Raiders 26-12, extending Canberra's losing streak at home to six matches.
Former Warrior Bill Tupou spilt the ball with the line open in the 69th minute, effectively ending the Raiders' hopes of a come-from-behind win.
After landing four straight conversions, a Tyrone Roberts 74th minute penalty goal put the result beyond doubt as Newcastle registered their second win. Canberra have won two of their first six games.
Jarrod Croker opened the scoring in the ninth minute when the Raiders found numbers out wide and the centre dragged his opposite Dane Gagai over the line with him.
Newcastle hit back in the 16th minute through McManus after Joey Leilua slipped a pass close to the Raiders line.
Rookie Knights hooker Adam Clydesdale worked his way over from dummy half in the 32nd minute, after Croker failed to ground the ball for a no try from the video referee a minute earlier.
- AAP