The 26-year-old prop is out of contract with the Airlie Birds and rejected their offer of a new deal for family reasons.
"I have had a really special time with Hull and enjoyed every minute of it, not only around the club and with the team but meeting so many people around the city too, so I'm thankful for that," Moa said.
"The club and the fans have been great to me. Hull FC gave me an opportunity four years ago and I wouldn't be in this position now if it wasn't for them, the coaching staff and the players I have had around me.
"However, the good times in life bring with it some tough decisions. With my family moving back to Australia last year it was a tough call to stay in Hull for another year but it was one I was keen to do.
"Now though the time feels right to go back. I need to be closer to my partner and around my family so it's a move I make with the best wishes and with fond memories of the club."
Moa has made 78 appearances since making joining Hull from Cronulla in 2009 before his season was ended abruptly due to a serious arm injury.
Hull coach Peter Gentle said: "You never know, Sam might still make it back in time to play again this year should we have a decent run in the finals but, if he doesn't, he knows he has still had a great career with Hull FC and played his part.
"He is a popular player and a big part of the club, on and off the field, so he'll be missed I'm sure.
"We made him a significant offer to stay with us, which he gave plenty of consideration to, however, he has decided the time is right to head home and we understand the rationale behind that."
- Hull FC