Matai has played 12 times for the Kiwis since his international debut in 2006, including at the 2008 World Cup, but confirmed earlier this year he would swap black for blue.
He had been on the outer with the Kiwis last year, failing to even make the train-on squad for last year's end-of-season test with Australia, but the 29-year-old has recaptured the sort of form that makes him one of the best and most damaging centres in the NRL.
He would have been handy to have available with Gerard Beale injured, Jerome Ropati lacking match fitness, Konrad Hurrell unavailable (Tonga) and doubts over Dean Whare, who is the subject of doping investigations.
Matai has played a leading hand in Manly's recent resurgence, which has seen them win their last five games, and scored 10 tries in 18 games this season. His combination with Kieran Foran, Justin Horo and Jorge Taufua on the left edge is one of the most potent in the competition - something the Warriors will be mindful of on Sunday when the two teams meet at Gosford - and together with Jamie Lyon represent arguably the best centres combination in the NRL.
"It was a tough decision for me [to choose Samoa] but it was a family decision," Matai said. "I spoke to my parents about it and they couldn't be happier. Seeing me play for their home country, where they were born and raised, was a way for me to say thank you to them for the sacrifices they have made for me and my brothers and sisters.
"They were over the moon. I couldn't be happier, too. Pleasing my parents that way, I think is the best way I can show my gratitude to them for what they have done for me."
His parents still live in the home in Mt Roskill, Auckland where Matai was brought up and where his disabled brother, who needs 24-hour care, lives.
Matai has been something of a recruitment officer for Samoa, trying to convince players to join him at the World Cup, and he even sounded out former schoolmates Sonny Bill Williams and Thomas Leuluai.
Both are likely to play for the Kiwis but Samoa are still likely to assemble a dangerous outfit featuring the likes of Roy Asotasi, Eddy Pettybourne, Jeff Lima, Junior Sa'u, Antonio Winterstein and David Fa'alogo.
They have been drawn in the same group as the Kiwis, France and Papua New Guinea and Matai expects them to make a statement at the tournament.
"I think we are capable of beating one of the bigger sides in England, New Zealand or Australia," he said. "We can now pick the whole side from NRL players and that goes a long way to beating those teams."
Manly still have a smattering of players who won premierships in both 2008 and 2011 but have also added the likes of Horo, Taufua and Brenton Lawrence and are building good momentum in sight of the playoffs. They are now third on the ladder and have won their last five by a combined margin of 198-68.
The Warriors also have momentum on their side, having won seven of their last nine, and will be desperate to win to keep in sight of the top eight.
Warriors side to play Manly at Bluetongue Stadium, Sydney, on Sunday (4pm): Kevin Locke, Ngani Laumape, Dane Nielsen, Konrad Hurrell, Manu Vatuvei, Thomas Leuluai, Shaun Johnson, Sam Rapira, Nathan Friend, Ben Matulino, Feleti Mateo, Simon Mannering (c), Elijah Taylor. Interchange: Suaia Matagi, Jacob Lillyman, Todd Lowrie, Dominique Peyroux.
