there is definetly some reefs near Yanchep/Two Rocks that would be breaking nicely . I have heard so many stories of a secret/notsosecret spot somewhere north of Two Rocks, and also legends of somewhere near Burns beach/Quins Rocks ...
There is so many reefs North of perth that there are obviously some that would be sailable.
Probably still unknown (apart from those in the know) just because they may never break in summer, and the winter sailing crowd is still pretty small.
Let me know if you hear any other Myths and Legends, I 'll come and bust or hopefully prove them .

there are many many many breaks north
many reefs and many beaches
if you got time and an imagination you will be rewarded
seek and ye shall find... knock and it shall be opened unto you
but go with a buddy coz you will be all alone

with a 4 - 5m swell i figure pretty much anything up there will be breaking. Luck .