The Cowboys have agreed to release Tonga, 27, from the final year of his contract to allow him to sign for another club.
“I have nothing but good things to say about the Cowboys, my time with the club, the coaches I have worked with, the players I have played with and the staff here,” Tonga said.
“I’ve had a great time playing footy with the Cowboys and look forward to ending off this season on a high note.
“I have decided that I want to move away from Townsville and that’s the only reason why I’ve come to this decision.
“I’d like to thank the club for letting me chase other options and hope our members and supporters can understand why I’ve decided to leave.
“It’s been an honour to play for the Cowboys.”