The AFL and Optus Stadium are investigating an "aggressive and loud" fan that has allegedly attacked Fremantle forward Michael Walters after the game.

West Coast and the Dockers played in the 58th edition of the Western Derby on Saturday, with the lowly Eagles taking the chocolates by 37 points.

Following the dismal performance from the men in purple, Channel 9's Tom Morris reported that a fan was abusive toward Walters after the game.

It is understood that the remarks weren't racist but "considered attacking in nature and over the top."

Fremantle has offered to assist where needed.