A Dubbo junior, Costin made his début for South Sydney Rabbitohs in 1993, before switching to Western Suburbs Magpies for the next season. Costin spent 4 seasons with the Magpies. In 1994, he made 8 appearances, but managed to score seven tries in the last five games of the year. His try-scoring form continued in the start on the 1995 season, scoring 6 in the first five games. Towards the end of the year he was moved from centre to five-eighth. In 1996, Costin was a regular in the centres, playing in every game of the season. In the round 12 match against the Gold Coast Seagulls, Costin scored three tries in one match. That year the Magpies played semi-final football, but were knocked out in their first match. Midway through the 1997 season, Costin made a late change to join Eastern Suburbs. He went on to play in 2 semi-finals with the Roosters that year, but did not play in the Preliminary Final that they lost. In 1999, Ben Kennedy and Costin were sent home from training after being involved in a drinking session. There were claims that the pair had taken drugs, and Costin confessed to taking an illicit substance, but Kennedy claimed to have spat the tablet out. Subsequent drug tests for both returned negative. As Super League VI champions, the Bradford Bulls played against 2001 NRL Premiers, the Newcastle Knights in the 2002 World Club Challenge. Costin was selected for the interchange bench in Bradford's victory. Costin played for the Bradford Bulls at centre in their 2002 Super League Grand Final loss against St. Helens. In the 2003 Super League season, Costin was a nominee for the Man of Steel Award and was the 4th highest try scorer, despite missing the last 5 games through injury. He left the club at the end of the 2004 season and went into semi-retirement. After his retirement from professional football, Costin played and coached with Souths Logan in Brisbane.
Playing career
In 2010, Costin was coach of the Queensland Residents side. In 2018 he coached the Coffs Harbour Comets in the Group 2 Rugby League. An injury crisis at the club saw Costin playing in the semis that year at 46 years old. Costin took over as coach of Fiji in 2019. His first match in charge was a 58-14 win over Lebanon.
