Cahill made his debut for Australia in a friendly against South Africa in March 2004. He scored his first two goals for the side in a win over Tahiti at the 2004 OFC Nations Cup. In the next game of the tournament, he scored his first international hat-trick against Fiji. His 12th international goal, and Australia's first in an Asian Cup, was against Oman on 8 July at the 2007 AFC Asian Cup. His 29th international goal against Costa Rica on 19 November 2013 equalled fellow Australian Damian Mori's record, which he surpassed on 5 March 2014 with two goals in a friendly against Ecuador. During a 4–0 win over Bangladesh on 17 November 2015 in a 2018 World Cup second round qualifier, he scored the first three goals of the match, marking his second and final international hat-trick. Excluding those two hat-tricks, Cahill scored twice in an international match on ten occasions.