Lane Poole Reserve near Dwellingup seems to have some nice Estuary fishing. There is a national park camping site. But you might need to book it before you go.
Micheal, you can camp on the beach at Preston beach just past Mandurah. Do you have a four wheel drive? Floreat beach is another good location just south of the drain sun up or sun down Go onto the Surfcasters website and pick up fishing locations.