The Auckland-born ex-New Zealand Warriors player has signed a contract extension keeping him at the club until the end of 2018.
Tupou, 25, joined Wildcats from Canberra Raiders last year on a contract lasting until the end of the 2015 campaign.
After impressing then-coach Brian Smith and chairman Michael Carter he later signed a new one-year deal and that has now been extended.
Tupou has made only three appearances for Wildcats this season due to an ankle injury, but is getting close to fitness and could make his return away to Widnes Vikings on Friday.
“I am very happy,” Tupou said. “It was a very easy decision for me and my partner. I came here around June last year and everything just clicked from there.”
Tupou was signed as Smith, who had just joined the club, began to rebuild his squad for last year’s battle against relegation in the Qualifiers.
He has yet to play under current boss Chris Chester, but said: “He is a good coach.
“He has brought the team together and by the looks of it everyone wants to stay because of him, everyone is performing because of Chezza.
“He has seen a few of my games and he wants me to stay on.
“It is very pleasing for me and my partner and I am looking forward to staying on for the next few years.”
Chester said: “I am delighted Bill has put pen to paper, he’s an important part of our team and I am looking forward to seeing him return to action in the next week or so.
“Once again this shows us how important it is for us to keep all of our best players to move forward as a club.
“Bill will play a big part in this team over the next couple of years.”
Tupou is the third player to sign a contract extension in the past two weeks, following in the footsteps of Chris Annakin and Mika Simon.
Meanwhile, Hull FC captain Gareth Ellis has been charged with grade B reckless striking following a tackle on Simon in his team’s 46-28 First Utility Super League win at Wakefield two days ago.
Ellis – a former Wakefield player – was not charged over an earlier challenge which left Anthony Tupou with a mouth injury and concussion.
The match review panel’s report stated: “Player making legitimate attempt to tackle opponent. Player’s shoulder does not make contact with head of opponent. Accidental clash of heads in the tackle.”