Training with the first grade squad over that summer earned Kostjasyn a spot in the club's World Cup Challenge squad and a subsequent medal as part of the winning team. He was later signed by the club on a two-year contract.
He then went on to make his National Rugby League début in the 2010 NRL season. He went on to play 7 games that season making his début in Round 1 of that season. He scored his first try for the club in Round 14 of the 2010 season.[citation needed]
2011
In 2011, Kostjasyn was a more regular member of the Melbourne team, playing 12 matches, starting in three of them. Known for his versatility, played games at five-eighth, hooker and lock in 2011.
2012
Kostjasyn played in 14 games in the 2012 NRL season, deputising for injured teammate Cooper Cronk in the halves at times. In Week One of the 2012 finals series against the South Sydney Rabbitohs, Kostjasyn made his first finals appearance. He was named 18th man for the Melbourne Storm in the 2012 Grand Final.
On 26 July 2012, Kostjasyn signed a two-year deal with the North Queensland Cowboys for the 2013 and 2014 seasons.[10]
2013
In 2013, Kostjasyn mainly played the hooker for the Cowboys, rotating with Ray Thompson. He made 20 appearances for North Queensland in 2013, including the semi-final against the Cronulla Sharks, which he started at hooker.[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.
2014
In February, Kostjasyn played in the Cowboys' championship-winning Auckland Nines side. On 2 July, he re-signed with the North Queensland on another two-year-deal, keeping him with the Cowboys until 2016.[11] He missed just one first grade game in 2014, due to broken ribs, starting 23 games at hooker.
2015
On 4 October, Kostjasyn was a member of the Cowboys' Grand Final winning side, coming off the bench in the side's 17–16 victory over the Brisbane Broncos. The Grand Final was his 100th NRL game.
2016
On 21 February, Kostjasyn was a member of the Cowboys' World Club Challenge winning side, coming off the bench in the side's 38–4 victory over the Leeds Rhinos at Headingley Stadium. He played 26 games in his final season at the Cowboys and represented Country Origin for the first time in May.[12][13] In October, he signed a 2-year contract with the Newcastle Knights starting in 2017.[14][15]
2017
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. He stayed overnight in hospital and was released on 8 January.[16] On 25 July, he announced his retirement from rugby league due to breathing difficulties from the injury.[17]
Post playing
Kostjasyn after retiring from the NRL he revealed that his injury compromised his breathing and changed his voice.[18] In 2018, Kostjasyn became the coach of the Newcastle Knights reserve grade side.[19] In 2020, Kostjasyn was appointed as the teams Development coach.[20] On 3 October 2024 Dolphins (NRL) announced via social media that Kostjasyn would join the team as an assistant coach.[21]