You can use any HSDPA modem, as long as it supports the 850mhz frequency (as that's what Telstra use). For maximum speed, make sure it supports 7.2mbit or greater - most of the cheaper USB HSDPA modems only support 3.6mbit, which is nice, but NextG supports 7.2mbit and even 14.4mbit (but those modems are only sold by Telstra as no-one else in the world has a network that fast! I think).
The other caveat is that many of the branded modems are SIM locked to that provider: e.g: a 3 dongle will probably be locked to 3. There are ways to unlock it using software, but I'm not sure how to do it. Hopefully another MacTalker will help you and me by divulging such money saving information, haha
But yeah, if it's SIM unlocked and supports HSDPA 850mhz - all you need to do is put in your Telstra SIM and away you go.