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Your backpack is the thing you should buy last. Because it depends on what you put in it.
You can only have a real idea of how large and comfy your pack should be when you already have everything else. I've seen countless people buy backpacks that ended up being too small or too large for their gear, not comfy enough in regards to the weight they carry, and with options not suited to what they need. You don't want that.
That is, for your "real" backpack, the one that's expensive and that you're going to use for years. In the meantime, to get started, I'd recommend a cheap one at about 30L. Shouldn't cost more than 20€$, less than 30AU$ if I'm correct. I don't know what brands of "discount" sport gear you can find in Australia, but look into that.
It's a great thing to buy cheap stuff, because you end up having all sorts of problems with it, which teaches you what to look for in that particular piece of gear, and you end up with a more precise idea of what you need. Like whether you need something bigger or more comfy, several compartments, hiking pole loops, mesh pockets, compression straps...
Only then will you look into getting a good backpack.
