Sonny Bill Williams believes he's doing the right thing after walking out on the NRL to pursue a rugby union career in France, according to close friend Anthony Mundine.

Mundine spoke to Williams on Sunday night as the AWOL Bulldogs superstar was at Singapore airport bound for Europe where he was expected to take up a two year, $3 million deal with Toulon despite having four years to run on his NRL contract.

"I told him it was pretty crazy here and that there was a lot going on but he sort of anticipated that before he left," two-time former world champion boxer Mundine told Fairfax.

"He said he genuinely feels in his heart and in his mind that he's done the right thing.

"The only thing that was really bugging him was that he didn't have the opportunity to explain to his boys - and I don't know if that was boys as in teammates or boys as his close mates in the team - and his hardcore fans at the Dogs and in rugby league in general how he felt."

Williams told Mundine that if someone who was close to him was in the same situation he would show them all the support he could and he hoped in time his close friends and fans would understand and be there for him.

Mundine said he drove Williams to Sydney airport on Saturday and was planning to visit the Kiwi international in Toulon after his fight against Japan's Crazy Kim in Newcastle on Wednesday night.

He said Williams had given his decision to leave a lot of thought and had been encouraged by suggestions he could become an All Blacks great.

Toulon's coach is former All Blacks captain Tana Umaga and their biggest signing for the season starting next month is All Blacks loose forward Jerry Collins.

Mundine indicated Williams saw the move as a new challenge and also an escape from unwelcome media in Sydney.

"He's a bright boy and I know that he wasn't too happy with the club and with the game and with certain elements of the media," Mundine said.

"I think a lot of people have made outlandish statements that don't really need to be made.

"They're only thinking about themselves but they don't think about Sonny and what he must have been going through."

"I think people need to wait to hear why he's in this situation.

"He was unhappy and maybe he had new challenges, maybe the new challenge for him was to become an All Black.

"He'd spoken to me about these things before so maybe he felt like he'd had enough of rugby league and that he had new challenges in rugby union and this was the first step."

Mundine said Williams had asked him to drive him to the airport when he left.

"He was pretty cool," Mundine said. "Obviously he was a bit jittery because he would have probably been fearing the unknown but besides that he was pretty talkative. We had a couple of laughs but, at the same time, he was pretty serious as well. You could tell he was on edge but he'd obviously thought it through."

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