Best way to check if its a machine on your network causing the traffic is this - unplug the router from the power for a few hours. Then plug it back in and check the Orcon usage logs for that period. If theres data showing, then something funny going on, either with your line or Orcons logging. (I think there is a slight delay with it showing)
If theres no data showing, then its def something on your system/s causing the data use.