In 2004, Cornes was moved to centre-half back. The new position worked wonders as Cornes began to single-handedly dominate games, earning himself his first All-Australian award, and finishing third in the Brownlow medal. The move to centre-half back was a key reason for Port Adelaide's dominance, which culminated in their first Premiership in 2004, defeating the Brisbane Lions by 40 points. He had a key defensive role on Brisbane Lions' forward Jonathan Brown during the Grand Final, restricting him to just one behind for the match.