Thank you for this very interesting question.
Section 116 of the Australian Constitution allows for freedom of religion in Australia by preventing the parliament from making laws which prevent the establishment or practice of religion in Australia. This prompts your question of how ‘religion’ is to be defined.
Effectively there is no complete legal definition of ‘religion’. The constitution itself does not go into any detail of what ‘religion’ should mean. There has also never been a law passed by the Australian Parliament or a ruling made by the High Court of Australia which absolutely defines religion. In fact, when ruling on the subject judges have been careful not to establish one standard legal definition of 'religion'. As Justice John Latham wrote in his ruling on the case of Adelaide Company of Jehovah’s Witnesses Inc. v Commonwealth in 1943, “ It would be difficult, if not impossible, to devise a definition of religion which would satisfy the adherents of all the many and various religions which exist, or have existed, in the world”.