Do not leave the Grand Barrier Reff without some close encounter with kangaroos, wallabies, and koalas. Although in theory you could do these things when in Sydney, the Sidney Zoo is much bigger and doesn't allow you the level of access and interaction with animals that these small places do. There are two excellent wild life habitats in the Grand Barrier Reef area that are very intimate- the bigger one in Cairnes and the smaller one in Port Douglas. Since we stayed in Port Douglas we visited that Wildlife Habitat Port Douglas of course, and it was wonderful. The admission was app. $25 (less for kids), and there is an additional cost for koala photos, but it wasn't exhuberant.
There are certain times only during the day when you can get your photo taken with a koala (so called "koala presentation"), and when we were there the koala presentation took place at 10:45am. If this what you are interested in, plan your visit accordingly. This visit will take few hours, not longer, it's a perfect size habitat that allows you to enjoy, learn, have fun but its not overwhelmingly big, and you can do something afterwards.
Pregnancy for koalas lasts 35 days, and then their babies (also called joys) spend another 6 months in the pouch getting stronger, feeding on mother's nipple. It's completely unclear to me how they survive on eucalyptus diet. You will see a lot of wallabies and kangaroos, and in theory it seems easy to tell the difference, but I somehow never really could. Wallabies are like small kangaroos and there are some other things, but don't ask me to name them. They are very friendly and will come to you to feed them.
One rather scary thing, in addition to crocodiles which makes you miss the small Florida gators, is enormously big spider web. It almost forms a roof and most people try to pretend they are not afraid wen walking underneath, but everyone speeds up at least (while some literally run). Don't miss this amazing place and the opportunity to have your photo taken with a koala.