NRL boss David Gallop says rugby union must join rugby league in taking a stand against Sonny Bill Williams' shock decision to walk out on the Bulldogs and attempt to switch codes mid-season.

The Bulldogs forward reportedly flew to France on Saturday to start a two-year deal with cashed-up and newly-promoted French first division rugby union side Toulon.

Williams' exit in the first year of a five-year deal has left the NRL in shock and his Bulldogs teammates and club furious with seven rounds still remaining this season.

Gallop said he would fully support the Bulldogs' plans to take out an injunction to stop Williams playing rugby.

But he also called on the International Rugby Board to pledge their support, believing there could be ramifications beyond the NRL if Williams did not honour his contract.

"We'll be calling on the IRB to step in and get involved," Gallop told the Nine Network.

"This can't be good for either of the codes.

"Contractual stability is important to both and if they don't get involved then they're condoning a form of international piracy."

Bulldogs chief executive Todd Greenberg agreed the issue was one not just for rugby league but Australian sport as a whole to deal with.

"It has some big ramifications and we need to get the buy in of a whole range of codes," Greenberg said.

"This is not good for anyone in any sport that someone can take a contract, sign it and then go and find somewhere else to go.

"We need to work hard together in unison to make sure that happens."

Greenberg said the Bulldogs would be willing to talk to a European rugby club about a possible compensation deal if Williams was to switch codes but said he knew nothing of the Kiwi Test star's plans.

Greenberg said he was disappointed he had not heard from Williams and only got confirmation the player had left the country when he phoned Williams' manager Khoder Nasser after the news broke.

Gallop refused to blame Nasser for Williams' walkout.

"These are young men but they are adults and they've got to make their own decisions," Gallop said.

"Life is about making choices not matter what your advice is from your agent or anyone else.

"Sonny Bill Williams made this decision to walk off without notice.

"... It goes way beyond contracts and money, what we're talking about here is someone walking out on their teammates mid-season.

"It flies in the face of everything that kids are taught about team sport and that's the most disappointing aspect of it."

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