Though this is to late for the OP, I want to comment: Kangaroo Island is worth a few days of visit. K.I. was never infested with mainland imports, so interesting wildlife not generally seen outside of reserves can be seen (e.g. echidnas) and the scenery is spectacular.
For the diving, several days of diving will reveal some of the greatest algal species diversity in the world, seals, dolphins (and the opportunity to swim with them), over 150 fish species (some of them endemic), leafy and weedy sea dragons or course, and the opportunity to dive in a pristine and well-kept environment. Those from California, eat your heart out - abalones in 30 fsw, big bugs / crays in Oz / lobsters, you name it, it's waiting here. Water may be warmest in February / March, and if you can swing it when the juvenile leafies are out and about, bonzer!
Kangaroo Island Diving Safaris is the place, but my experiences with them were when Jim and Josie Thiselton were about. Jim guided National Geographic and other photographers to the leafies and other wildlife, offered farm stays at their place, etc. The new KIDS is owned-operated by Mark Snadden, and I am sorry to say I don't know anything about him so I can't comment. I will vouch for his statement ". . . unique Leafy Sea Dragon, Australian Sea Lions, Fur Seals and swim with Dolphins in a pristine wilderness area with spectacular views . . ."
Fly, take the ferry or swim
- you won't be sorry you went. In 54 years of diving, this is one of the memorable dive spots in my logs (frankly, moreso than the GBR, as I prefer Fiji, Vanuatu, etc. to the IMO less than pristine GBR).