Born in Sydney, Fainu's rapid rise through the NRL ranks has landed him at the forefront of discussions for both Tonga and the Kiwis.
Another spanner was thrown into the works when the 20-year-old met with NSW Blues coach Brad Fittler and declared an interest in playing State of Origin for the Blues.
Fainu spent time with the Tongan squad ahead of the Pacific Test in June last season after representing the Junior Kiwis in 2017 on account of his mother's heritage.
He was selected for a Junior Kiwis' squad in 2018 but did not play.
The international tug of war is likely to come to a head before New Zealand host Tonga in Auckland on June 22.
"I'm available for either Tonga or New Zealand but I'm unsure what I'm doing, even NSW if they want me," Fainu told NRL.com.
"It's something I need to think about and make a decision soon. No one has approached me from either country, I haven't got a call or anything like that but I'll talk to my family and we'll see.
"My dad is Tongan and both my parents are supportive of my decision."
Making the case more intriguing for the Guildford junior is the influence of Tongan international Addin Fonua-Blake and Kiwi prop Martin Taupau on him at the Sea Eagles.
Fainu admitted Fonua-Blake had been in his ear about committing to the Tongan side but that the Junior Kiwis experience had left him torn on his next decision.
"Addin just said to me to follow my heart and do what's best for me, he said only I can decide where I want to play," Fainu said.
"Junior Kiwis was great and I learnt a lot about my mother's heritage and representing that side of my family."
Fainu has earned high praise since bursting onto the scene as a teenager at the Sea Eagles under former coach Trent Barrett last season.
The Guildford product has scored three tries in seven appearances in 2019 and continues to put pressure on regular No.9 Api Koroisau at the Sea Eagles.
His service at dummy half is even more impressive given he spent his junior days as a playmaker alongside rookie Cade Cust.
"I had a couple of bad games in the 20s in a row and got benched, our coach thought he would use me at hooker and I haven't looked back," Fainu said.
"I was nervous playing under Des this season. He comes across as very intimidating to everyone but he's the opposite to us as players and a very good coach. Api there too he's helped me transition at hooker a lot."
Photo NRL. Caption: Manly hooker Manase Fainu