Christina is correct. Aussie weather is just opposite to HK. Winter usually starts in July to Aug. Cairns s/b a little warmer than Goldcoast as it is in far north of Aust. If you have time, u can also enjoy skiing in Blue Mountain, Sydney.
I think both cruise has not much difference. The point is where you want to go, I remember Green Island & Outer Barrier reef is quite worth to see. But don't choose Fritzroy Island as it is small have nothing to see. Have a nice trip!
Depends on what brand of milk powder your son is drinking. If it's Japanese brand, you can't find it in Aust. They have S-26. As for the nappies, other baby stuff and baby food, you can get it (Hernz) at most supermarket, Target and chemist everyway. So you don't really need to worry about that. I do suggest you bring along your BB車. Most of the places in Aust. are baby friendly.
I understand your concern. It's like we went to a 3 day trip when my son was 6 month old. We brought all the stuff which filled up the whole car. We even brought the milk bottle steamer.
Anyway, you can leave those stuff for later use. Their water is very clean. You can even drink the tap water. But still, it's better to mix the water with the bottle one. You never know, your son may not use to it.
Remember to go to the Target. They have a lot of baby stuff and clothes to choose from. But made in China if u don't mind.