I can cite you the USA regulatory definition:
(7) Active ingredient means any component that is intended to furnish pharmacological activity or other direct effect in the diagnosis, cure, mitigation, treatment, or prevention of disease, or to affect the structure or any function of the body of man or other animals. The term includes those components that may undergo chemical change in the manufacture of the drug product and be present in the drug product in a modified form intended to furnish the specified activity or effect.
(8) Inactive ingredient means any component other than an active ingredient.
21 CFR § 210.3(b)
But I have no idea how to do legal research on Australian labeling requirements. Probably the website of whatever agency you have that corresponds to the US Food and Drug Administration will have links or information?