All Black star Sonny Bill Williams' return to the NRL has finally been approved after a drawn out process. 

Williams signed a one-year deal with the Sydney Roosters on November 13 but World Cup winner’s boxing career complicated the process for salary cap auditor Ian Schubert to approve the deal.

Williams, at his welcoming press conference with the Roosters back in November, appeared disinterested in his return to League, which made it clear he was only doing so to honour a handshake promise to the club's chairman Nick Politis.

There were some concerns Williams would walk out on his Roosters deal if the NRL didn't register the contract quickly, but after a tense period of waiting, the Roosters have their man.  

"By the nature of Sonny Bill's other sporting commitments there have clearly been some unusual circumstances around the contract including his direct negotiation of the contract through a legal representative," said Interim chief executive Shane Mattiske.

"It has been the salary cap auditor's absolute responsibility to review all aspects of the payments in the contract and the negotiations leading up to the contract.

"Ian Schubert has done exactly what he is required to do throughout this process and his task has been made more challenging by deficiencies in the Roosters' documentation of the negotiations, which did not fully comply with the NRL's player contract and remuneration rules."

He added that the Roosters can expect a fine in the new year for failing to follow regulations.

"There are a number of assurances on the part of the club and in relation to Sonny Bill's commercial agreements that are being relied upon in this process and that will be monitored," Mattiske said.

"Given the unusual features of this contract it was necessary for Ian to engage a solicitor to assist with the investigation which included interviews of a number of persons and a consideration of various documents.

"Ultimately, a report of the investigation was furnished to the Commission along with advice from Senior Counsel.

"The Commission has also determined that the club should face a sanction ... the amount of this breach will be assessed following a recommendation in the new year."