I would suspect that the US versions of the firmware aren't going to work for you. US has SiriusXM radio and a slightly different FM radio band than Australia. (Yours starts at 87.5MHz and the US starts at 88.0MHz.) I'm not sure what else might be different as well.
It probably won't allow you to even start the update. And if it does, it could possibly brick your head unit.
Now, that said, if you want to take the chance, the current firmware for the 2015 Outback Navigation head unit FB644US and FB664US is NU9022_USB_1st.exe and NU9022_USB_2nd.exe. Those files are available on my Google drive at the link a few posts back. I am in no way suggesting that you try it, but it's available for use at your own risk.
I would suggest that if you think you need the update, find out the equivalent of the STIS (Subaru Technical Information System) for Australia and look around there to see if updates are available. Or ask your Subaru dealer.
If you want to PM me your VIN, I have about a day left on my STIS access. I'll punch it in and take a look for you.