The Manly Warringah Sea Eagles have been hit hard by the NRL for salary cap breaches totalling $1.5 million over the past five years, with past and present club officials - including Immortal Bob Fulton - earning the ire of head office.

The Sea Eagles were fined $750,000 and two officials - chief operating officer Neil Bare and former CEO Joe Kelly, who is now the boss at the Sydney Roosters - received 12-month bans, while Fulton will need NRL approval to take up any official role in the game again.

Manly will also be forced to play the remainder of this season and the 2019 Telstra Premiership with a salary cap of $330,000 less than other clubs but they have not been docked competition points for the breaches, which involved 13 players over a five years period.

Sea Eagles coach Trent Barrett was given an official warning to improve his understanding of governance issues around the salary cap, but NRL CEO Todd Greenberg said there was no evidence he had been part of the breaches, which revolved around the misuse of third-party agreements.

Greenberg said an investigation, which kicked off last July at the invitation of Sea Eagles chairman Scott Penn, had uncovered a series of third-party agreements organised by the club and offered to players during contract negotiations but not declared to the NRL salary cap auditor.

The Sea Eagles had wanted to dispel rumours of salary cap rorting at the club which arose duringa NSW Crime Commission investigation into match-fixing allegations but the NRL found 800,000 documents, including text messages and emails, about the club's use of third-party agreements.

There were no findings of wrongdoing by the providers of third-party agreements but the Sea Eagles had breached the salary cap by offering additional payments to players during contract negotiations and later organising the TPAs for them.

Sydney Roosters announced that Kelly was considering his options and Easts Group CEO Scott Bennetts had been appointed to his role in the interim.

"They all had one aim, to attract players to the Manly club by offering additional, undisclosed payments outside of the salary cap," Greenberg told a press conference at Rugby League Central on Monday.

"In simple terms, there are undisclosed payments for remuneration at the time of negotiation which takes players off the market, and that is against the rules.

"I can understand the disappointment that would be felt by 15 other clubs, who potentially would feel they had missed out on a player during a negotiation. That is why the measurements of penalties we are placing today are significant."

More than a dozen current and former Sea Eagles officials and players were interviewed but Greenberg was critical of the level of co-operation provided by the club.

"One of our concerns from this investigation is that the club had every opportunity to come clean about the activities and to cooperate fully with the investigation. It did neither," he said. "Yet the evidence obtained by the integrity unit is indeed compelling.

"The other clubs have every right to be angry, not just that the game's image has been tarnished by another salary cap saga, but because they may have missed the opportunity to sign players because of Manly's undisclosed deals."

The Sea Eagles are the latest club to be hit with heavy sanctions for major salary cap breaches following Canterbury Bankstown Bulldogs in 2002, Melbourne Storm in 2010 and Parramatta Eels in 2016.

Greenberg warned that any further clubs or officials caught cheating the salary cap would face significant penalties.

"Our salary cap is the main reason we have the closest competition in Australian sport and we will not tolerate any attempt to breach it," he said.

"Every club needs to be aware that any attempt to cheat the salary cap will eventually be detected and the club involved will be heavily sanctioned."

The NRL has imposed the following penalties:

A fine of $750,000 with $250,000 suspended if the club undertakes appropriate governance changes to ensure there is no repeat of the breaches;

A $660,000 penalty to be applied to the club's salary cap. This will affect the club's salary cap this year and next year;

Bare has had his registration suspended for 12 months but will be eligible to return to the game on January 1, 2019 if he undertakes appropriate governance training;

Sydney Roosters CEO Joe Kelly, who was previously employed at Manly, has had his registration suspended for 12 months but will be eligible to return to the game on January 1, 2019 if he undertakes appropriate governance training.

Manly and the officials can appeal the sanctions to the NRL Appeals Committee.