Interesting to see the word "gin" - rarely heard now. It has become quite (intentionally) derogatory and many dictionaries now say it related to aboriginal women used for sex by white men. There was no such connotation when I was young - my mother, grandmother used it with no offence intended - they would talk about bucks or gins coming to the station etc. It was simply a female aborigine (as compared with lubra, which seemed to have the specific meaning of wife - "Jackie's lubra"). The origin of the word "gin" seems lost in time. Strange how words become derogatory over time.
One suggestion is "It is derived from the Dharuk word diyin, meaning woman" - the Dharuk or Darug being the Sydney people who would have been the main group to first have contact with Europeans. Myall also comes from their language (Mayal), originally just meaning someone from another tribe but becomimg a general term for someone living a traditional life style. Used to also be common when I was young - haven't heard it in decades (nor gin).