The 20-year-old - who made his NRL debut in 20187 - spurned an approach from Kiwis coach Michael Maguire and confirmed his allegiance to Tonga, NRL.com reported.
Fainu follows a pathway created by four players - Jason Taumalolo, David Fusitu'a, Sio Siua Taukeiaho and Manu Ma'u - who defected from the Kiwis for Tonga on the eve of the 2017 Rugby World Cup tournament.
Manly prop Addin Fonua-Blake, who played for the Kiwis at the World Cup, switched to Tonga in 2018.
Fonua-Blake helped convince Sydney-born Fainu, who has Māori and Tongan heritage, to commit to Tonga.
"My first option is Tonga so I've spoken to Kristian [Woolf] and told him how I feel," Fainu told NRL.com.
"I've spoken to Madge [Maguire]. He called me ... and I told him I would love to play for Tonga.
"If I don't get picked [for Tonga], I'll just wait – because I don't want to go to New Zealand as second choice ... that would be disrespectful to them."
Woolf - Tonga's coach since 2014 and a current Newcastle Knights assistant - is due to name his 23-man squad on Wednesday afternoon.
Fainu, a specialist hooker who can play other roles, has already attended some Tongan training camps at Fonua-Blake's urging.
"[Fonua-Blake] has spoken to me a lot about it," Fainu said.
"He's always saying how good it would be to run out together for Tonga.
"I've actually been to a couple of camps and they are the best camps I've been to.
"They are enjoyable to be around – some good people involved with Tonga."
Tonga were beaten semifinalists at the last World Cup, where the Kiwis crashed out to Fiji in the quarterfinals.
The Tongans drew over 26,000 fans to their historic first ever match against Australia at Auckland's Mt Smart Stadium last October.