I've called TPG but the lady on the phone wasn't entirely helpful, and managed to talk me into buying a new modem (even though it was only 10 dollars, but still). In short, my NBN box's UNI-D1 lights are completely off on the back after an electrical storm swept through briefly today, and my modem only has two lights on at all: power and WLAN. I've tried resetting my modem, resetting the HFC NBN box, new cables, and just about everything else you care to mention. I found an old NBN box lying around from this address and tried that. I note that it didn't connect (unsurprisingly) but the light in the UNI-D1 port lit up, and my router had 'internet' and 'WAN' lit up as well as WLAN and power.
So, as far as I can tell, this is a problem with my NBN box? Could this be sorted ASAP as I will be losing work tomorrow as I cannot work from home without a stable, reasonably high speed, internet connection.