Originally from country Western Australia, Seaby was recruited from West Perth at pick 22 in the 2001 AFL Draft by the West Coast Eagles. He made his senior AFL debut in Round 4, 2004 against the Brisbane Lions and became a regular player that season due to injury issues with senior ruckman Michael Gardiner. While Gardiner's understudy Dean Cox rose to prominence, Seaby served as a secondary ruckmman into 2005, occasionally being used as a tall forward option with some success. In early 2006 his chances of maintaining his position received a boost with Gardiner receiving an indefinite suspension from the club due to disciplinary issues.