Sullivan moved to Hamilton after finishing high school, where the Chiefs – via their development team – helped him with his recovery from his knee injury.
[13] During the provincial union's end-of-season awards function, Sullivan was named the NPC Back of the Year.
He didn't travel with the New Zealand Secondary Schools team to Australia for that year's three-match international series.
[18] The following year, he missed out on a spot in the New Zealand Secondary Schools team due to the knee injury he suffered while playing for King's College.
[21] In June 2022, Sullivan – who is of Ngāti Kahungunu descent – was named in the Māori All Blacks squad to take on Ireland during their 2022 tour of New Zealand.
[23][24] On 10 October 2022, Sullivan was named in the All Blacks XV squad for two matches against Ireland A and the Barbarians during their Northern Tour.