Im a complete noob when it comes to long-distance bike riding. So im not sure what would be the best
Im just thinking cause of the terrain that it may require a mountain bike
New Zealand has some of the best roads around, so if you're planning to stick to roads as opposed to off-road trails, you won't need a mountain bike. What you will need, for loaded touring, is very low gearing to climb some of the hills they have over there and that's a simple matter of replacing chainrings and cassettes on a triple.
BTW, when you say you want to ride around Australia, do you mean all the way around? It's about 16000km by road depending on route, so it could take you a while.
A MTB for touring is not a bad idea as it can make maintenance easier and cheaper.
Check out Cycle Life Online which is the blog of a couple that toured South Amercia from South to North on MTBs. Silas rode the same bike round South East Asia a couple of years earlier as well.