Unfortunately for the consumer (but understandable from a training standpoint), Telstra only provide support for modems supplied by Telstra. Also, it does appear to be an issue with this particular modem and working with the NBN (which I wouldn't have expected a shop assistant to know about unless they owned one and had it on the NBN).
Typically, there shouldn't be a problem with them NBN and any modem that supports VDSL2 on a FTTN connection (and in the vast majority of cases they operate with no problem).The problem for owners of hardware that isn't supplied by their ISP is that there is no duty of care owed by the ISP (and there shouldn't be - the costs would be enormous), plus when the NBN arrives, you ultimately have no choice but to change over.