The only work around I've found so far is switching your country to UK. You can leave your phone language and regional format Australia, but the country has to be set to UK.
This is yet another reason why Microsoft absolutely should remove the "same speech/country" requirement for Cortana. It's stupid, frustrating and feature breaking. I tried to set up "Hey Cortana" for my mum, but I'm not going to use a workaround for her just in case it breaks something else. This stuff should be SIMPLE. (Also I find the UK Cortana voice atrocious, but that's another story).