Been away for a while so this post slipped past.
Quite a few good spots in WA that will meet your temp criteria. Exmouth and Coral bay as has been raised. The navy pier in Exmouth is sometimes touted as the best pier dive in the world. Murions and Ningaloo reef generally are pretty nice.
If you've got a few dollers to spend Rowley Shoals further to the North east are pretty amazing. All trips are liveaboards out of Broome. I can't remeber the operators but I'm sure google will find them.
Houtman Abroholos islands of Geraldton are very cool along with 17th century wreck sites.
alot of the coast south is nice. Not alot north of Perth is dived so you've got a lot of coast to explore.
You're biggest problem is wind. December is really windy right along the coast. I lived in Shark Bay, 350km's south of exmouth and we had 19 consecutive days where the wind didn't drop below 30kts for a second about that time of year. That was a bad month but just to give you a heads up. On the other hand May June is majic. You get days where the water is so glassy calm you can't tell where the sky ends and the sea starts.