On a hunch just contacted a friend who has just started getting the same problem. Lives in a very small town and does not have NBN availabel so has been using mobile. Seems his problem started the same day as the nbn rollout in nearby Millicent AND evry day since the rollout he has been getting an error message when he starts his pc that "Someone has the same IP address as him". A message like this from an ISP ( as this error should not occur on an ISP NAT) also points toward a NBN related error.