Gday Steve,
Dec and Jan in dunsborough sounds like a tough gig....yallingup, smiths to margret river and beyond are good spots, have a look at a local map with bays n beaches, see which ones would be good with a solid 15-25/30 knot sou wester, have a few stubbies, sleep in and get out around early arvo and you'll be smiling. I dont think there's the kite restrictions like there is around the city locations, but excercise caution as always or someone will have a cry, but theres plenty of good beach with few or no people, and generally no shortage of swell.
There is usually plenty of wind in geographe bay, it will get you to africa in about four days.
I'll be down and around that way a bit over summer kiting, PM me if you want to get involved.
Bully