NRL chief executive David Gallop has softened his stance on Sonny Bill Williams returning to play in Australia, saying the renegade backrower would be welcomed back if he apologised.

The Bulldogs star, who turned 23 on Sunday, is believed to be making his way to Toulon to meet with officials over a lucrative rugby union deal, but Gallop says the door remains open for a return to the NRL.

"We would take him back - absolutely," Gallop told Fairfax.

According to the report, Williams was considering a switch to boxing after becoming increasingly frustrated with life at the Bulldogs, before opting to leave for France last Saturday.

If this is the case, Gallop says he is willing to listen.

"From day one, I've thought it seems like he's troubled," Gallop said.

"I would like to understand why he has done this. If he had a change of heart then certainly we would want to sit down and listen to him.

"We would want to see him back in the NRL and we would want to see him happy. If Sonny was to say he's sorry for what he did and admit that he made a mistake, we would definitely be a party to hearing him out."

Gallop's latest comments are in stark contrast to those he made a week ago, when he first spoke of Williams' departure.

"If Sonny Bill were to play overseas, certainly the door would be closed on his returning to the NRL at any time in the future," he said at the time.

Despite his mystery whereabouts, Williams has been named in New Zealand's preliminary squad for the Rugby League World Cup in Australia, which begins on October 26.

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