Can I ask, why you want to dive Australia? Are you going to be there and want to dive it while you're there or are you going there specifically to dive it?
Australia has "different" rules. The Spirit of Freedom has been reported to have a lot of issues (the same as I experienced and wrote about 10 years ago are still reported to exist). There are national laws in Australia that control diving, that you may not like.
I strongly recommend subscribing to Undercurrent.
Spirit of Freedom: Be very careful - See 393 traveler reviews, 568 candid photos, and great deals for Cairns, Australia, at Tripadvisor.
www.tripadvisor.com