Aside from device ID, or maybe if you’re using a billing address as something from Australia, I can’t see how they would be, you’ve stated location services are off, VPN will mask the ip.
I didn’t even need to use a VPN in my cases between Canada and the US, changing to an account that was in the region that allowed the purchase was all I had to do.
That being said if I changed my Canada account to use the billing address of my US residence instead of my Canadian residence when on the Canadian region, I would get region locked I found, so if you have a primary payment method on file using your AU address you could try temporarily removing or modifying it to have a different address
As someone who jumps between the US and Canada (since on the border) I’ve found that, Amazon will block purchases from Canada to the US and visa versa, it doesn’t care about ip, it uses your accounts region, Canada and US have seperate regions in your account settings that you need to do. Sadly this means you would need two different accounts. One for CA and one for US. It’s likely the same deal with audible
yea that was my next question, if the title was actually available in the country selected, I had that issue with Netflix searches constantly