1) Clothes are kind of expensive. I wasn’t fully aware of this until I dropped $110 on basic black Levis and $40 on cheap casual shoes for work the first week. Normally I say “buy it when you get there” but this is a legitimate exception.
2) It’s too easy to set up the small things. I was irrationally worried about getting a bank account / tax number / SIM card. All of this was the most convenient I’ve ever had in any country. I mean, you could buy ready-to-go smart phones over the counter at the post office for under $100.
3) You should probably book-in before arriving in any city. Backpackers hostels were busy in every city I visited in Australia, like crazy busy, even in low season, and waiting until the day before means you might end up paying for a $100+ “single” in some dodgy place.
4) Getting the initial WHV is super easy, extending or getting another type of visa is a Massive Pain. Every place has someone who knows someone who got a shady farmer to sign off for $1000 at the last minute, but that system is quickly becoming backpacker urban legend with little basis in reality, for many reasons. If there is any remote possibility you would want to extend, or go PR, get on this early, so you’re not in a high pressure situation later.