The Rabbitohs played with 14 men for roughly 20 seconds of Friday night's 32-6 victory at Suncorp Stadium, with Tom Burgess taking part in tackles and running the ball up before retreating to the interchange bench.
The incident took place in the 53rd minute when the Rabbitohs made two interchanges at the same time, Jai Arrow and Davvy Moale replacing fellow forwards Tevita Tatola and Burgess.
The NRL's investigation into the incident revealed the error occurred when prop Moale handed his interchange card to a Rabbitohs club official rather than the NRL's interchange official and entered the field before Burgess had come from it.
Players are required to hand their card to the NRL official to prevent such breaches from occurring.
"With player Arrow and player Moale entering the field at approximately the same time, there were 14 Rabbitohs players on the field at the same time for between 15 and 20 seconds," an NRL statement read.
The error did not affect the ultimate outcome of the game, but has been lashed by NRL CEO Andrew Abdo, who confirmed the onus was on clubs to ensure the interchange process was run correctly.
"It shouldn't happen, it's not good enough," Abdo told The Continuous Call Team the day after the game.
The $15,000 fine includes punishment for a team-sheet error in the same game.
The Rabbitohs named Tatola to start at prop in the final team list submitted by the club, only to select bench prop Hame Sele in his place on game day.
The Rabbitohs have five days to respond to the breach notice.
