North Queensland has been fined $20,000 for breaching concussion guidelines, prompting an NRL warning to clubs who put their players at risk ahead of what looms as a brutal finals series.

The Cowboys became the third club to cop the hefty fine this year following an investigation into an incident involving forward Tariq Sims in round 20.

NRL head of football Todd Greenberg said Sims should have been taken from the field in the 69th minute when he showed "clear signs of suffering a head injury".

The NRL has suspended $10,000 of the fine - just as it did when imposing penalties on the Wests Tigers and Canterbury earlier this year.

The Cowboys will pay the additional $10,000 if there are any further breaches of the rules in the next 12 months.

Greenberg warned the rules would be enforced "stringently" during the finals series if clubs put their players at risk in search of a premiership.

"The welfare of our players is too important to risk because of any game of football," Greenberg said.

"So clubs will be expected to put aside the importance of the finals and remove players from the field for assessment if they suffer head knocks.

"This fine (on the Cowboys) should act as a timely reminder to clubs that we expect the NRL's concussion rules to be followed if players suffer head knocks.

"We believe the concussion rules have generally been enforced by the clubs this year and we are adamant that this must continue through the finals series."

AAP