AFL defender Mitch Brown is under investigation after an incident in a WAFL practice game that left a player with a broken jaw.

The West Coast backman was playing for East Fremantle against South Fremantle on Saturday as he returns from injury.

South Fremantle youngster Mason Shaw suffered a broken jaw in an incident that allegedly happened behind play.

There was no video footage and the umpires at the game lodged a notice of investigation.