Hey Alex, good to see you'd like to visit Western Australia for kiting. Our kite season in Perth begins in October so by December and January you'll be right in the middle of it (usually best and most consistent winds). Anywhere around Perth, North or South would be just great and you'll find a mix of flat water spots, waves spots and choppy water spots. Kiting in Margaret river is a must, especially in Augusta (30 min drive south of Margs), you'll find a few KM of flat water and waves at the back, all in the same session (see pic below). At that time of year, you should get 3-4 days of kiting a week at the very least and if we get a good (normal) season you should be able to kite nearly every day. Good thing about Margaret river and Augusta is it's not very crowded and there is a ton of space. Kite Beaches around Perth can get busier especially during week-ends and holidays. Also consider Lancelin in your trip.
As for Bali, at that time of year from what I know it would be the off season for kiting so you'd be lucky to get any wind. Also, it would be very hot and humid. I went there in November and it was almost unbearable for a Canadian like myself. In Dec-Jan it'll be the hottest time of year in Bali (40 degrees + ) and it quite often gets saturated with humidity and no wind... If you're strictly looking at kiting in Bali I would give it a miss for that time of year. Two months won't be too much to discover all of what Western Australia has to offer in terms of kiting, especially at that time of year.
Enjoy!
