When I got home tonight, the behaviour of the internet link was similar to that of a capped connection, but I have 15Gb to spare, so was a bit perplexed.
I gave TPG a call, they did a few checks then asked me to reboot my modem, so I removed it's power for 10 seconds, then waited the 5 minutes until my line came back up.. as usual, the line took an age to sync, then rock solid, speedy intermenets again... and I only waffled at the poor tech support chick for about half that time before deciding to just leave the support ticket open until I call back... thanks Patricia, we're working again.. TPG rock (and I'm so sure she's sitting there in the call centre, mta paused, reading this post).
So yeh, she explained the connection had clocked up quite a few errors and that the modem reboot resets all that.. nice to get useful, informative support.
For anyone interested, I'm running my second officeworks-purchased Netgear D834G, a modem I get great results out of, and always behaves snappily both from internal and external requests. It also includes a 4 ports switch, which our linux (mta) server hangs off, and a wireless router, which I am communicating from this laptop via.
I would suggest any issues that caused the outage, in this instance, are more likely the Telstra dudes I saw in the hallway the other morning, than any unusual degrade in performance of the router I know and love.