Wainui was a member of the Takapuna Grammar School 1st XV in 2011, 2012 and 2013 and was awarded North Harbour's 2013 Māori Colts Senior Player of the Year whilst still in school.
He represented New Zealand on the world stage after playing for the Champion New Zealand team in the Under 20 Rugby World Cup in Italy. The team played England for the title, winning 21–16.
In 2015 he was signed by the Super Rugby franchise the Crusaders after impressing in the centre and wing position for Taranaki.
He made his debut for the Crusaders in round one of the 2016 Super Rugby competition, starting in the outside centre position against the Chiefs.
Wainui made 7 appearances for the Crusaders in 2016 and continued his growth in 2017.
Ahead of the 2020 Super Rugby season, Wainui has been named for the Chiefs squad. He has obtained 35 caps so far.
Wainui has represented New Zealand as a part of the Māori All Blacks.
|2014 - present|
|Maori All Blacks
|2015 - present|
|2018 - present|
|New Zealand U20's
|2015 - 2015|
|2015 - 2017|