Wainui's Chiefs teammates Brad Weber and Anton Lienert-Brown shared emotional tributes from America, where they are touring with the All Blacks.
"Can't quite put into words how much this one hurts. Always said publicly that Sean deserves everything he gets because he works so hard for it. But he doesn't deserve this. That first day back at Chiefs HQ next season is gonna hurt walking in and not seeing you there brother. RIP," Weber wrote on Twitter.
"A warrior on the field and one of the great humans off it. My heart hurts. You'll be missed massively but never forgotten my brother," Lienert-Brown said on Instagram.
Former All Blacks star Sonny Bill Williams was among the many others in the New Zealand rugby fraternity to pay tribute to Wainui.
"My thoughts are with Sean Wainui's family. Especially his wife & young child. Although I didn't play alongside him, I could always feel his mana coming up against him as his opposition," Williams wrote on Twitter.
The All Blacks also shared their condolences on social media: "We are heartbroken right now. Sean, you were an inspiration and will never be forgotten. We extend all of our strength and aroha to your friends and whanau … Kia au tō moe."
Wainui was born in the small settlement of Whatatutu near Gisborne and raised in Auckland before attending Takapuna Grammar School where he was a stand-out as captain of the first XV before making his debut for Taranaki in 2014, while still a teenager.
Wainui played 53 games for Taranaki before shifting to Bay of Plenty this year. He also played 44 games for the Chiefs, nine for the Crusaders and represented New Zealand Māori since 2015.
Of Ngāi Tūhoe, Ngāti Porou, Ngā Ariki Kaipūtahi and Te Āitanga a Māhaki iwi, he was a vocal advocate for te reo Māori, proud of his Māori heritage and especially passionate about his whānau.
On the field, Wainui was remembered for his try-scoring ability and one of Taranaki's most celebrated players.
The Chiefs took to social media to farewell "one of Rugby NZ's tallest tōtara trees".
"One of Rugby NZ's tallest tōtara trees has fallen. To you Sean, our Rangatira, we farewell you to the outspread arms of the multitudes who await you beyond this earthly realm. You leave us here bereft and drown in sorrow as we weep for you. Rest well in peaceful repose."
Taranaki rugby union historian and statistician Matthew Shaw said Wainui was one of Taranaki's "more celebrated players".
"He certainly had a great career down there."
Wainui made the New Zealand under-20s in Taranaki in 2015 and was their 51st Māori All Black, "something he'll be in the record books" for, he said.
"He'll be very much missed."
Bay of Plenty rugby historian Brent Drabble said Wainui was a "top bloke".
"He only just came to us this year – he played one game for my club Whakarewarewa here in Rotorua before he went across and played against New South Wales and scored four tries for the Chiefs."
Drabble said he was in a "state of shock". He said he only met Wainui once but it was clear "what a neat kid he was".
Wainui made his debut for the Bay of Plenty this year in the game against Tasman in August and scored a try. His last game for the Bay of Plenty was last month in the Ranfurly Shield match against Hawke's Bay.
