One of Sonny Bill Williams' closest friends, Bulldogs lock Reni Maitua, says he knew "something was up" in the days leading up to the Kiwi backrower's shock mid-season walkout on rugby league.

Maitua said he had not heard from Williams since he fled Australia on Saturday, reportedly to take up a multi-million dollar French rugby union deal.

But he admitted to sensing his good friend and teammate was troubled in the lead up to his dramatic exit.

"I am one of Sonny's closest mates and he didn't tell us any circumstances or anything like that," Maitua said after the Bulldogs 30-0 loss to the Dragons.

"But I knew sort of something was up.

"I didn't know he was leaving but I just knew something was going on, he wasn't himself for a couple of days there and then it all unfolded.

"He just sort of left and it's really sad to see him go.

"I think it's extremely disappointing but I love him as a mate and I'll always stick by him, as long as he knows that." Maitua defended Williams against a wave of criticism arising from his departure, believing his teammate would not have left so suddenly without a good reason.

"I'm sure Sonny would have had reason why he had to leave now, he wouldn't have just walked out," Maitua said.

"I'm not prepared to call him a dog until I know the circumstances.

"I think people making those comments, I think they're just hurt to see him go.

"He's such a good talent, he's such a good player for the NRL.

Maitua said he would stick by Williams, just as he did with now Roosters forward Willie Mason, who controversially left the Bulldogs last season.

"I love the kid," Maitua said of Williams.

"I signed at the club at the same time as him, I'm not prepared to go out and bag him because I'd be a hypocrite.

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