Hi Scooty,
Did you / Telstra ever sort this out? I've found exactly same problem on Teltra Gateway Pro (V7610) as you are reporting. If the Port Forwarding, external and internal ports match, it works fine (eg 3389 to 3389) . But when Port Translation attempted it fails everytime (eg 3400 to 3389). Very annoying, because it means that each PC being used for RDP access in the network needs a hack to the Registry to change Listening Port and Firewall !
Telstra tell me that Port Forwarding to different port number is supported on this model model (with same firmware)
Telstra also tell me that they have tested and it works for them.
Well it's been 7 weeks now that I have been reporting this to them, and still not working !!!!