It's literally the best place on the east coast for flatwater windsurfing ... Soooooooooooooo good!
It works all winter (the locals can be more specific). I assume June/July would be the best. I spent all of last June there...sailed nearly every day with +20 knots perfect sideshore from the top of the island. It's a decent length boatride out from Cairns each day but you are sailing on the reef and it's so spectacular.
Looks a bit like this.