I was up there in august for 3 weeks last year, it's a bit cold but got about 3 kites per week, flies were fine, i have also been up in march to april quite a few times, had some good kiting and no probs with flies (must have been lucky cos it can be a nightmare). If its about wave riding it's a great time to go as not so many people on the tombstones wave. The flat water spot at Gnaraloo bay is great, but if it goes wrong it's a long way across the bay.

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If it's all about flat water i would not go to Gnaraloo, just go straight to Exmouth, but if it's about wave riding, then it's the top place in WA. But you need to be good, it's a pig of a launch site with reef/rock everywhere. December is better, but then again you have way more people including windsurfers to compete with. In march/april you pritty much get the wave to yourself, but you do risk also not getting any wind.
Also gnaraloo is a secret spot so don't' go telling anyone about it