Killen made his international debut for the New Zealand senior team during the 2000 OFC Nations Cup against Tahiti, three months before his first senior appearance at club level. The "All Whites" finished the tournament as runners-up, after losing 2–0 to Australia in the final. He was also in the squad for the 2002 OFC Nations Cup, which New Zealand won after beating Australia 1–0 at the Mount Smart Stadium. He was further named in the 2003 FIFA Confederations Cup, and featured in the defeats to Japan and Colombia at the group stage.