The Aboriginal Australian people are most commonly referred to as Aboriginal Australians. We should make tags based on the more populist terminology. The term Indigenous Australian is technically correct as well, but normally includes both the Aboriginal people and the Torres Strait Islanders. The latter are Melanesian and considered distinct from the mainland Aboriginals.
The term "Aboriginal" is traditionally applied to only the indigenous inhabitants of mainland Australia and Tasmania, along with some of the adjacent islands, i.e., the "first peoples". Indigenous Australians is an inclusive term used when referring to both Aboriginal and Torres Strait islanders.
I suggest that questions on Aboriginal mythology should use one of aboriginal-australian or australian-aboriginal or australian-aborigines, with the others marked as synonyms.
Questions on the mythology of Torres Islanders could either be tagged with their own torres-islande, or use a more generalmelanesian tag.
The indigenous-australians tag could perhaps be used for questions on both groups, or question on general indigenous Australian beliefs without specifying a subgroup (e.g. does any indigenous Australian group believe that...?), or possibly a synonym to a main aboriginal tag.