Hey Carl29,
Sorry to hear of the issues with the NBN affecting your TV reception 
As this is working when you have the NBN turned off it may be that there is an issue with the modem or where this is currently situated. Are you able to move the mode away from the TV turner at all and see if this does improve the experience at all?
If not then there may very well be some cabling that is damaged and affecting this service. For this I would recommend contacting the NBN Assurance team on 1800834273 - IVR > 3 which is available 24/7.