He made his Currie Cup debut at the age of 19, playing at 8th man in 2008, and was picked on the bench for the Barbarians side to play Australia at Wembley, later that year. He made his Super 14 debut for the Stormers in 2009, playing on the wing, but suffered a knee injury against the Lions at Newlands which put him out of action.
His full-time return to the top flight was off the bench against the Griquas on 8 August 2010. The following week he started at openside flank (no.7) versus the Blue Bulls. Köster had Province's highest tackle count (24 tackles) against the Bulls, while his 11 ball carries was second only to Duane Vermeulen and he was one of only three WP players to secure a turnover.
In October 2012, it was announced that he would join Bath. He scored his first try for his new club against Italian team Calvisano in the Amlin Challenge Cup.