There are a few lake spots where people go speed sailing on occasion, but they've got very specific wind directions that they work in. The ones that I know of also involve a loooong walk in with your gear, so you've got to be keen.
Generally in summer we get decent ssw seabreezes most afternoons (supposedly

), which makes for really good sailing on the coast and river, not so good inland.
If you're moving to mandurah, most people sail on the estuary, I believe, and wave sail on the coast., rig up at novara boat ramp at the estuary, and talk to some locals before you head out. the estuary is pretty good sailing but there are some submerged rocky areas to look out for....