Hiya! I've been working in marine biology since my undergrad, and I've just started my PhD in a marine biological field. From what I understand, what you're taking out of your PhD are skills, not necessarily specialisation (although that of course is part of it all). So, if you wanted to change fields, that shouldn't be a problem.
I can't really help with moving over to Oz, but it might be nice to actually get in touch with some scientists over there who share similar interests, and speak to them about it. They can probably provide advice, as can international offices in their universities, as they would have seen it all before.