You don't mention which part of Australia you are from so not sure if water, sea life activities are something that would be unique to you. We discovered after doing WDW after a cruise once that it is much nicer to do WDW first and the cruise after, far more relaxing after the go-go-go of WDW. New Years is fantastic at WDW, yes there are crowds but park and fast pass planning really helps. We also don't do Magic Kingdom on New Years Eve, instead we go on the 30th and enjoy the New Year Eve show which runs that night as well. By Jan 2 you will have run of the parks, crowds have thinned dramatically. We also like to take a park break and visit some of the resorts on property like Animal Kingdom Lodge, Trails End dining, the Boardwalk area and Downtown (Springs). Because there is so much to do in each park, we don't get park hopper and instead spend our day in a single park. With a longer stay we have the chance to repeat our favorites.
Are you renting a house and visiting the parks from there ? On property we enjoy eating at different places within the different resorts, all have very nice holiday decorations. Our long stays are in Villas where we have a kitchen and end up doing some meals at "home" just because it is easy.
Now I have to say I love the idea of doing 16 days or so in Orlando and then doing a Bahama's Cruise followed by a Caribbean Cruise, this might just have become a goal of mine. Our daughter has been on 11 cruises and loves it as much as visiting the parks, of course she always wants both not one or the other, our next one is in 8 weeks. Hope you have a great time with whatever you decide and look around the boards to get some great tips about doing the parks as well as the cruise.