Williams, 28, was omitted from the initial 24-man squad announcement having made himself available, but appeared to have a change of heart.
“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.”
The news is a disappointment for Harris, 21, who enjoyed a fine debut season in the NRL, missing just one match for the Storm, and making his debut in April’s test match.
He was nominated for the Dally M Rookie of the Year award, and also played in the World Club Challenge win over Leeds.
“I was very much looking forward to going, however I understand the decision that has been made and hope I get another opportunity to play for the Kiwis in the future.”
The NZRL invited Harris to join the squad as part of the travelling party, but Melbourne, with full support of their player, declined the opportunity.