[6] In Round 8 ANZAC Day match against the Brisbane Broncos, Koroisau scored his first NRL career try, and the game's opening try, in South Sydney's last minute 28-26 penalty goal win at Suncorp Stadium.
[15] In Round 1 of the 2016 NRL season, Koroisau made his club debut for Manly-Warringah against the Canterbury-Bankstown Bulldogs, starting at hooker in the Sea Eagles' 8–26 loss at Brookvale Oval.
[16] However, he was dropped to New South Wales Cup in favour of Matt Parcell after the Sea Eagles lost 22–36 to the Wests Tigers in Round 2 at Leichhardt Oval.
He was recalled to the team for their Round 5 match against his former club South Sydney following an ankle injury that ruled out representative halfback Daly Cherry-Evans.
[citation needed] Koroisau made 13 appearances for Manly in 2018 as the club endured a horrid season on and off the field narrowly avoiding the wooden spoon by 2 competition points.
Following injuries to halves Nathan Cleary and Jarome Luai ahead of game three of the series, Koroisau made his debut for New South Wales in the number 14 jersey.
[25] On 22 July, Koroisau was placed under investigation by the NRL after allegations were made that he invited a woman into the NSW Blues bubble during the series, against strict biosecurity regulations.
[28] Koroisau played a total of 18 games for Penrith in the 2021 including the club's 2021 NRL Grand Final victory over South Sydney.
[33] On Thursday the 8th of June[34] In round 15 of the 2023 NRL season, Koroisau suffered a broken jaw in the clubs loss against the Gold Coast and was ruled out for an indefinite period.
[35] In round 25, Koroisau kicked a goal from the side line with less than three minutes remaining to win the game for the Wests Tigers 24-23.
[36] Koroisau played a total of 21 games for the Wests Tigers in the 2023 NRL season as the club finished with the Wooden Spoon for a second straight year.
[37] Koroisau played 22 games for the Wests Tigers in the 2024 NRL season as the club finished with the wooden spoon for a third straight year.