Gallen, the former Cronulla Sharks and New South Wales Origin captain, also returns to the ring for the first time since 2022, when he beat fellow former NRL player Justin Hodges by unanimous decision.
The enforcer boasts more in-ring experience than Williams, with a record of 15-2-1.
Gallen’s notable fights include stunning former heavyweight champion Lucas Browne with a first-round knockout in 2021 and falling to current WBA heavyweight title contender Justis Huni in the 10th and final round of their bout that same year.
The pair have been back-and-forth over negotiations to settle their differences, with both claiming the other fighter has been dodging them as recently as 2022 and beyond.
The 39-year-old Williams, who is four years younger than Gallen, may find an advantage with the shortened length of rounds, which was the final sticking point in negotiations between the two parties.
Professional boxing rounds are typically three minutes, with the two-minute stipulation notably seen in the controversial Jake Paul-Mike Tyson bout in November
Williams and Gallen are expected to earn $1 million each in a 50/50 split, in a fight that will garner plenty of interest across New Zealand and Australia, with a broadcasting rights battle brewing on this side of the Tasman.
The undercard will see cruiserweight fighter David Nyika return to the ring for the first time since his devastating knockout defeat against IBF champion Jai Opetaia in January.
The Kiwi’s opponent has yet to be confirmed.
Elijah Fa’afiu travelled with assistance from Manuka Phuel.
