I suspect it
IS Factory Unlocked. I think AT&T offers it, but not in a specific configuration as a carrier branded device. Does it say AT&T anywhere on it? At any rate RM-984 is the only variant listed at gsm arena and they are pretty good about showing model variants locked to carriers. The other major carrier in the US is T-Mobile and the 830 lacks the right frequencies to play nice with their service - hence in the US 830 becomes AT&T by default. That is different from purpose built.
If you used a known IMEI verify site and it returns as Unlocked that means Factory Unlocked, they can't track SIM Unlocked by IMEI so far as I know. I trust Swappa to check IMEIs for me.
http://swappa.com/?ref=mobilenations...pUvbUpU2888279
Here is the spec:
Nokia Lumia 830 - Full phone specifications
Here are the Radio Frequency by carrier tables for Australia:
Mobile country code - Wikipedia, the free encyclopedia
Voda matches up so the sellers story checks out. Telestra looks good, Optus is fine, you can match up carriers on the table linked above...
Here is a table just on LTE:
List of LTE networks - Wikipedia, the free encyclopedia
I think this phone is a go.