How is it connected, WiFi or Ethernet? I'm assuming WiFi which would indicate to me your TV is taking a little while to find and connect to the signal when it turns on. To check this, next time your TV turns on and it doesn't connect straight away, go into the settings and manually reconnect to the WiFi. If this works, it's more then likely a TV issue and not an Optus/Internet/Modem issue
Ah. I get that the most common cause for an issue is something that just happened (i.e. the maintenance) however I suspect your NBn is working fine but that the down time cause some config to reset and that all you need to do is fix that issue.