The 28-year-old dual international performed a u-turn on Tuesday when he informed coach Stephen Kearney of his availability for the tournament beginning later this month.
A Kiwis squad of 24 for the tournament that didn't feature Williams had been announced hours earlier.
Williams followed up with a tweet wishing the team well and commenting on his need for a break.
However, Kearney confirmed in a short statement on Wednesday that Williams was now back in the selection frame.
"Late on Tuesday, Sonny Bill Williams expressed his desire to play for the Kiwis in the Rugby League World Cup.
"Our thinking had always been that if Sonny was available, he would be part of the team. We are now exploring options around Sonny's possible inclusion in the tournament. This would require ratification by the World Cup organisers."
Kearney confirmed that Williams' inclusion would have to come at the expense of a player already named as tournament rules allow for a maximum squad size of 24.
The NZRL statement said further comment would be made once had additional information.
Williams last played for the Kiwis in 2008, soon before leaving for a rugby union club contract in France.
He made a spectacular return to rugby league this year, leading the Sydney Roosters to victory in the NRL grand final on Sunday.
Speculation surrounds his sport of choice next year, with a return to New Zealand rugby reportedly being considered.