St George Illawarra coach Paul McGregor is facing a $10,000 fine after being issued with a breach notice for his attack on the NRL referees following Saturday's loss to Newcastle.
McGregor is the first coach this year to face sanction for criticising match officials, having called them "embarrassing and incompentent" after the match.
"In simple terms, the rule is that coaches, players and officials should not discuss referees and their performances in the public domain," NRL head of football Brian Canavan said on Tuesday.
McGregor's comments come a week after Canberra coach Ricky Stuart escaped a fine for launching his own tirade on match officials after his team's loss to Melbourne.
South Sydney counterpart Michael Maguire also put the heat on the game's whistleblowers for allowing play to continue despite claiming Adam Reynolds suffered a concussion on Saturday.
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Canavan said McGregor's comments crossed the line.
"We have been pragmatic in dealing with instances where comments have been made, but the competence and integrity of referees has not been questioned," Canavan said.
"But it is our view that these particular comments crossed the line. All coaches should be aware that they risk sanctions if they discuss the performance of referees."
The Dragons, in a statement, declined to say whether they would appeal the fine.
McGregor was livid at three key decisions that went against his side at McDonald Jones Stadium, including two calls that denied his team tries in the 21-14 loss to the Knights.
And while the defeat left them on the cusp of dropping out of the top eight, McGregor stopped short of saying it cost them the game.
He was adamant both calls were wrong.
"I think that anyone who knows rugby league would agree because the ball went back on the (Jason) Nightingale one and it should have been overruled and it wasn't," he said post-game.
"And (Trent) Hodkinson took a dive."
The Dragons, who were also slugged $50,000 this year for flouting concussion guidelines, have five days to respond to the breach notice.
