Калгари Хитмен
Postma has played major junior in the WHL for the Swift Current Broncos and Calgary Hitmen. After a 24-point season in 2006–07, he was drafted in the 7th round, 205th overall, by the Atlanta Thrashers in the 2007 NHL Entry Draft. The next season, he was traded to the Calgary Hitmen, where he recorded 14 goals and 42 points. In 2008–09, Postma emerged with a 23-goal, 84-point campaign, first among league defencemen, setting team records for single-season goals (surpassed Kenton Smith's 19 goals in 1998–99) and points (tied with Matt Kinch) by a defenceman.