An Ireland international and New South Wales Country representative, he played at hooker, but could also fill in at lock, and five-eighth.

He played in the 2006 NSW Cup grand final for Newtown who were the Sydney Roosters feeder club at the time against Parramatta.

[citation needed] In 2011, Kostjasyn was a more regular member of the Melbourne team, playing 12 matches, starting in three of them.

Kostjasyn played in 14 games in the 2012 NRL season, deputising for injured teammate Cooper Cronk in the halves at times.

[citation needed] At the end of the season, he was named in Ireland's 24-man squad for the 2013 Rugby League World Cup, playing 3 games in the tournament.

[14][15] During pre-season training with the Knights on 7 January, Kostjasyn suffered an accidental blow to his throat from Danny Levi.

He was required to be sent immediately to the John Hunter Hospital after suffering a fracture in the Cricoid cartilage in his neck.

[20] On 3 October 2024 Dolphins (NRL) announced via social media that Kostjasyn would join the team as an assistant coach.