New Zealand Professional Boxing Association president Lance Revill says they have no choice but to take the belt from cross-code star Williams as he hasn't responded to challenges for his title within a six-month deadline.
It follows the same decision by the World Boxing Association, which this week said Williams had forfeited its "international" belt, won with his controversial victory over South African Francois Botha in February.
Williams, 28, won the vacant NZPBA belt off Clarence Tillman nearly two years ago.
There were no challenges - which require the backing of a promoter - forthcoming until three months ago when unbeaten New Zealand National Boxing Federation champion Joseph Parker emerged.
Revill says details of the challenge, carrying winner-take-all prizemoney of $1.2 million, had been sent to Williams' manager Khoder Nasser via multiple emails and letters.
"We haven't heard a thing back and, personally, that's rude," Revill told NZ Newswire.
"He's not even said he's received the challenge and that they're going to think about it. If that was the case we'd hold off, but we've got no choice now but to strip the title."
Former Chiefs and All Blacks rugby player Williams announced he was staying for another season with NRL champions the Sydney Roosters one week after the Parker challenge was made.
He indicated then that boxing had dropped down his list of priorities.
Revill says it is unfair to deprive other fighters of a shot at the title, which will officially be made vacant next week.
He says it will be a shame if Williams doesn't return to the ring as he improved in his six unbeaten professional fights over the space of three years.
His grandfather, Bill Woolsey, was a New Zealand heavyweight champion.
"Sonny's got boxing in his blood and he learned very quickly. He impressed me a lot when I saw him spar before the Tillman fight, he showed me there he was more than a good athlete."
