I do not know about sources in Oz, but here are some USA source ideas:
N from KNO3, an agricultural fertilizer. Since ag ferts are sold in 50 pound bags, aquarium clubs and specialty aquatic fertilizer companies will repackage it, usually into 1 lb bags.
P from KH2PO4, another agricultural fertilizer, same comment as for N.
K from K2SO4, another agricultural fertilizer, same comment as for N.
Secondary fertilizers are usually found in the water:
Ca and Mg measured with the GH test, or separate test for Ca. If your tap water has at least 3 German degrees of hardness, then there is probably enough of each, Ca and Mg. If you are using reverse osmosis or distilled water, or your tap water is very soft (<3dGH) then the Ca and Mg are probably too low. GH booster is a general term for a nutrient supplement that supplies Ca, Mg and occasionally other minerals. For example, Seachem Equilibrium also has K.
Micros may be available separately, but are usually purchased pre-blended.
Iron is the most common mineral that is purchased separately, in chelated form (Yup! Another ag product).
I buy a blend called CSM+B that includes some iron, as well as the other micros. I do add more chelated iron.
CSM+B is not an ag product, but is a specialty aquarium product.
You might also look into hydroponic sources. Some of their products might work in an aquarium.
Here is a link to one supplier here in the USA. I do not think they ship to Oz, though. Anyway, look for something like this in your area.