South Sydney officials have lodged a complaint to Penrith after Sam Burgess was struck in the face by a spectator as he left the field after the Rabbitohs' 18-2 win over the Panthers on Friday.
A female supporter hit the England international just above the eye with a flagstick ashe walked towards the change rooms.
Burgess didn't sustain any injury and declined to comment on the incident.
However, Burgess' mother Julie vented her anger on Twitter.
"Would love to meet the woman (I'm being kind) who hit Sam with a stick as he was leaving the pitch. Sorry Penrith people but that's not OK," she wrote.
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.
