Wainui was a member of the Takapuna Grammar School 1st XV in 2011, 2012 and 2013. He was awarded North Harbour's 2013 Māori Colts Senior Player of the Year whilst still in school. Also an ex Takapuna Grammar Prefect and 1st XV Captain, Wainui 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.