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.
The talented and electric outside back Sean Wainui first represented New Zealand on the world stage after playing for the 2015 New Zealand Under 20 team, who won the World Rugby Junior World Championship in Italy, where New Zealand beat England 21–16 in the final. Returning from the Italy he joined the Māori All Blacks for the first time in 2015.
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.
In 2018, Wainui missed out on a Super Rugby contract but featured in the tournament after an injury crisis at the Chiefs and has since become a regular starter for the franchise.
By the end of the 2020 Super Rugby season, Wainui had been capped 38 times by the Chiefs and had played over 50 Super Rugby matches between his two franchises.
Off the back of a fantastic season, he was named the Chiefs' player of the year and was rewarded with a contract with the side through to the end of the 2022 season.
He heads into the 2020 Mitre 10 Cup with 43 caps for Taranaki.
|2014 - present|
|Maori All Blacks
|2015 - present|
|2018 - present|
|New Zealand U20's
|2015 - 2015|
|2015 - 2017|