This is a feature. If you're lucky, Tim from Telstra will see this thread and respond with his email address letting you know that he's here to help. But in reality this is lip service - no information of substance will be forthcoming.
For those in a similar position I managed to fix it. Probably the longest way round possibly but it worked so hey :)
1. Power off decoder 2. Power off modem 3. (after waiting for 15-20 seconds) Power on decoder 4. Wait for it to start up - it will complain about having no internet access 5. Power on modem - interestingly at this point it will seemingly semi-connect as the provisioning is obviously picked up but the Network details section in the menu still says Not Connected (oh and the reminder that the connection is back as alluded to by the no internet access message doesn't seem to ever come up) 6. Power off decoder 7. (after waiting for 15-20 seconds) Power on decoder
Perhaps I could have got away with just restarting the modem but wanted to ensure that the TBox forgot about the network and had to fully re-establish itself which is why I took the longer way round...