Ben Barba will serve his 12-match NRL ban at the end of his contract with Top 14 giants Toulon after his surprise code switch.
Barba, 27, was handed the mandatory suspension following a positive drugs test late last year, which led to his release from the Cronulla Sharks.
However, in January Barba - who was part of the Sharks' grand final-winning team - had agreed to a one-year deal with the club for 2017.
His contract was due to be submitted to the NRL for "approval and registration" when the Sharks announced the re-signing.
In a shock move on Tuesday, Toulon announced the signing of Barba until the end of their domestic season, but NRL chief executive Todd Greenberg said the talented full-back could not serve his ban while playing elsewhere.
"Ben does not have a registered contract with the NRL so he is free to make a decision to play in a different code with a new club," he said.
"But the NRL will not consider any contract for registration until he has completed his contract with other sports.
"The match suspension he needs to serve will only begin after he has completed his playing commitments elsewhere."