The Newcastle Knights may be forced to absorb a six-figure sum believed to be owing to star fullback Kalyn Ponga into their salary cap.

It's understood Ponga's father Andre raised the issue with the Knights last week, with reports suggesting the fullback hasn't been paid a six-figure sum by a registered third party.

According to The Sydney Morning Herald's Michael Chammas, the NRL are aware of the issue and currently investigating the Knights.

If proven true - and that the money is owed to Ponga - the Knights may be forced to take the hit on the chin, with the money retrospectively added to their salary cap.

The NRL's salary cap auditor signed off on the third party agreement at the time, but if it hasn't been paid, has the power to force the club to take the money into their cap.

"The NRL are tonight investigating a potential salary cap breach at the Newcastle Knights surrounding a third party deal in relation to Kalyn Ponga," Chammas said on Channel 9's 100% Footy.

"The Knights are aware of the situation and say it has nothing to do with them. The NRL have called a meeting late last Friday with Andre Ponga, Kalyn's father who actually raised the complaint with the NRL over the fact his son was owed a third party deal from 2020 to 2022."

It has been made clear the Knights have done nothing wrong in this situation, yet may be the party who winds up the worst out of it.

The NRL confirmed to the publication they are aware of the situation and are investigating, while the Knights have confirmed all third parties have been registered with the NRL, and that there are no current issues with the contract at the club.

Should any money from an unpaid third party deal be added to Newcastle's salary cap, it will mean the club continue to increase the value of the already highest-paid player in the game, with Ponga believed to be on a deal worth around $1.4 million per season after the Knights successfully fought off the Dolphins for his signature ahead of his new deal commencing at the start of the 2023 season.

Ponga is contracted at the Knights until at least the end of 2027.