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.

Let me know how the fishing is like there.

Thanks
Hyeshin

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.