The NBN uses the Telstra cable if you have HFC NBN. They can't touch Optus property. When you move across to the NBN an NBN tech will come out and install a new lead in for you (possibly using the conduit but not the cable in it). On the plus side your Optus cable keeps working while you get the NBN fully set up.
Are you sure you are getting HFC NBN? Check at the NBNCo Address Checker.