Capable of playing across the back row and at lock, the 26-year-old has established himself as a key figure for both the Crusaders and Canterbury, making his Super Rugby debut at the age of 19. A former New Zealand Under-20 international, Cullen progressed through the Crusaders Academy before making an immediate impact at Super Rugby level. His performances earned international recognition, earning his first All Blacks cap against Australia in 2020, while he has also represented the Māori All Blacks on three occasions. Since making his debut for the Crusaders, he has amassed 68 appearances.