RE: Windsurfing in Melbourne
Our Autumn is March to end of May, locally called "The Doldrums", when you might be lucky to sail only one day at the weekend.
Water temp begins to drop from the current 20 deg. C to about 8 deg. C in August.
Please spend some time hunting around the link given above: it's a mine of excellent information (and it should be: it's generated by a windsurfer who works for the Weather Bureau.
Hope this helps,