After signing a one-year deal with the Roosters on November 13, it's been a drawn out process for salary cap auditor Ian Schubert to approve the deal.
The dual-international is pursuing a boxing career on the side and has a fight against South African Francois Botha scheduled in February.
Williams represented the All Blacks this year before enjoying a stint in Japanese rugby, and there's every chance he will switch back to the 15-a-side code after his commitments with the Roosters are finished in 2013.
At his welcoming press conference with the Roosters back in November, Williams seemed nonplussed about returning to league, making 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.
Interim chief executive Shane Mattiske said signing off on the deal was a complex procedure.
"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 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."
Mattiske said the Roosters can expect a fine in the new year for failing to put all their cards on the table.
The Roosters failed to follow regulations in recording and supplying information to the ARLC, Mattiske said.
Schubert had to seek advice from a solicitor in rubber stamping the deal and Mattiske said the ARLC would also keep a close eye on Williams commercial agreements.
"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," he 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."