Hi, ShepNeil Have you tried looking at the telstra service status page? i tried to look for you but shepparton has several post codes.
There are to many reasons to list as to why your NBN may not be working but i can tell you, that your 4g working has nothing to do with your home NBN connection.
Have you tried doing the same check's that telstra got you to do yesterday when you rang, it may be the same problem.
mabe just turn your modem/router off and on again? sounds simple but you would be surprised how many times it works.
Rohan