australia, all you really need are the Ammonia, Nitrite, Nitrate and pH (high and low) tests that are in the master kit. If you have a planted tank, you may want to get other kits such as Phosphate, Iron, and so on and maybe even a GH and KH set but you can easily get by without them. If you want to know the GH and KH of your water and it's good info to have, just tank a water sample the the fish store and they should be able to test it for you. Just make sure they tell you the exact number and not that the waters "soft" or "hard".
Choc, that's the API kit I mentioned earlier. Probably the kit most used by the members here.