Where are you based and how important is flat water to you? If you want flat water in strong winds then Canton Beach near Toukley on the Central Coast is the go, however a weed fin is mandatory. Check out the pictures and video I made on a day with strong southerly winds a couple of weeks ago. It will give you a bit of an idea. There will be other windsurfers at Canton.
Unless you can't travel much I would not recommend Narrabeen Lake in a southerly. The wind will be very gusty and it won't be that much fun.
If you go to my youtube channel you will see a few videos of windsurfing at the spots you have mentioned.
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