It's not because of something inherent to Steam; each country is separate (not so much when it comes to trading, but that's neither here nor there). Lumping AU and NZ together is just something publishers tend to do, I would guess because they have to handle the staggered release themselves and doing NZ first seems a little pointless when it's such a tiny country and ahead by only a couple of hours (some poor soul has to wake up and kick off the process).