The answer will have a significant bearing on the makeup of the 24-man squad for their defence of the trophy this month.
Williams has maintained he will make a decision after Sunday night's grand final but doesn't have much time given the Kiwis squad will be announced in Auckland on Tuesday morning.
Coach Stephen Kearney has a number of phone calls to make on Monday morning, most principally to find out the health of those who played in the NRL showpiece, but the most significant will be to Williams' manager Khoder Nasser.
Kearney has been in regular touch with Nasser over the past month but he's still in the dark.
"Everyone has been waiting with anticipation regarding what Sonny is doing next in his football career," Kearney said. "I'm no closer than anyone else to getting an answer."
For Williams, it will come down to a question of legacy over money and boxing.
The 28-year-old has said he wants to be one of the greatest cross-code athletes of all time and winning World Cups in two different codes will help him achieve that.
On the flip-side, the off-season is the only time he can fully commit to boxing and he has a New Zealand heavyweight title to defend.
He's expected to return to rugby union next year to play for the Chiefs but nothing is ever guaranteed with Williams until it happens.
The Roosters second-rower is a guaranteed selection if he wants to play for the Kiwis, and his schedule and management team allow it, but there's almost of sense of resignation he won't be available. He would certainly add star power to what is already shaping as a good squad.
"It's one of the strongest squads we have had to pick for quite some time," Kearney said. "The calibre of the guys who miss out says a lot about the quality of this group."
The biggest questions seem to be around the makeup of the forward pack. The selectors are likely to plump for a 13 forwards, 11 backs split and will want some second-rowers and back-rowers who can also play either prop or centre.
That increases the stocks of players like Greg Eastwood, Frank-Paul Nuuausala, Elijah Taylor and Alex Glenn, but Kearney is cautious of picking too many utilities and not enough specialists. He said it happened for this year's Anzac test, although some selections were foistered on the Kiwis because of injuries and unavailability, and had an impact on performance.
The selectors will watch Sunday's showedown between the Roosters and Sea Eagles with a degree of nervousness because of the large number of Kiwis involved.
They so far have close to a full squad to choose from, with only Gerard Beale, Matt Duffie and Benji Marshall unavailable.
Issac Luke is likely to be the only specialist hooker, with Thomas Leuluai and Taylor providing dummy-half cover. Winger Roger Tuivasa-Sheck looms as the only debutant, although hard-running Roosters prop Sam Moa and Sea Eagles second-rower Justin Horo are also in contention.
The Kiwis will gather in Sydney next Saturday before flying to the UK a couple of days later. They will play a warmup against the Cook Islands in Liverpool on October 21 (NZT) before their opening World Cup match against Samoa on October 28 (NZT).
Possible Kiwis squad for the Rugby League World Cup starting on October 26 (NZT): Backs: Josh Hoffman, Jason Nightingale, Sam Perrett, Roger Tuivasa-Sheck, Manu Vatuvei, Krisnan Inu, Shaun Kenny-Dowall, Dean Whare, Kieran Foran, Shaun Johnson, Thomas Leuluai.
Forwards: Issac Luke, Jesse Bromwich, Sam Kasiano, Ben Matulino, Sam McKendry, Jared Waerea-Hargreaves, Greg Eastwood, Alex Glenn, Tohu Harris, Simon Mannering, Frank Pritchard, Jeremy Smith, Elijah Taylor.
