Williams, who won the World Cup with the All Blacks in 2011 then the Super Rugby title with the Waikato Chiefs in 2012, returned to rugby league this year, joining Sydney Roosters in Australia's National Rugby League.
The dynamic centre, who only signs one-year contracts so as not to be tied down to a club, has been the subject of several reports he could return to New Zealand in an attempt to make the 2015 World Cup squad, with Chiefs coach Dave Rennie suggesting as much earlier this week.
"The coaching staff and contracting team work very hard to have positive exit relationship with all our players and coaches," NZRU chief executive Steve Tew said at a media conference confirming All Blacks coach Steve Hansen's contract extension through to the 2015 World Cup.
"The door stays open and we communicate with them on a regular basis. But they know the rule, if you're not in the country then you can't be an All Black.
"If you want to come back then the door is always open.
"In Sonny's case, he's the same as everyone else. If he comes back fantastic. If he doesn't we respect that."
Tew said there had not been an agreement reached with Williams's management, while Hansen added he did not have a "handshake agreement" with the player to return to rugby.
The 27-year-old told reporters last year when announcing he would return to rugby league that he had made an agreement on a handshake to do so before he moved back to New Zealand from France.
Williams had been playing for Toulon in France's top division after he walked out on a five-year agreement with the NRL's Canterbury Bulldogs.
"He's got a plan in mind and he had a couple of options available to him. We just have to see what options he takes," Tew added.
"But there is no deal done. All we have said is that when he came here you were up front and you did a fantastic job for us and we would love to have you back some day."
Hansen, however, said there was no guarantee Williams, like other players who had left New Zealand with an aim to return in time for World Cup selection, would be included in his squad, let alone planning, until he returned to New Zealand.
"No problem with them leaving and no problem with them coming back," Hansen said when asked about players like Williams, centre Richard Kahui and flanker Jerome Kaino leaving for overseas contracts and then returning to play Super Rugby in 2015.
"All we've said is that if you come back and play Super Rugby and you're playing well enough then they will get selected.
"It's quite a simple process."
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