It is easy to spend a huge amount on food, I have to say. Even today, a small "nip to the shops" trip has cost me $80 for 2 bags of food. Granted, that did include 2 packs of Boursin (there is no substitute!) at ~$6 each, but most of our shopping trips are quite costly. I've never really worked out how much in a week but it would probably be around 3-4 trips a week, usually costing around ~$100. Fresh fish is viciously expensive if you buy only the local stuff (which we do, having seen docos about where Vietnamese fish/prawns are grown!)
Clothing too - especially for little children - vicious! Although having said that I did manage to get a couple of pairs of shorts for DS2 which were only $3 each; but that's a bit of a rarety IME. (Best and Less, iirc).
Books and DVDs also cost quite a lot more but if you don't buy many then they don't really count; and if you're not bothered about getting the latest releases then you can find discount shops for such things.
One of the things I really miss (but am not in Sydney, am 1.5h North of it) is the range of fresh fruit and veg - usually only 3 types of apple, for e.g. - I fall over in gratitude when I get back to Tesco and see the huge ranges of fresh F&V at the one nearest to my Dad.
Another thing to be aware of, even though it won't affect you just yet, is that you have to buy all school supplies for your DC, textbooks, exercise books, paper, pens etc. - so that's another cost. You also have to pay for all prescriptions for children too.
Remember too that a lot of the summer, many of us can't actually be outside because it's too hot!