Hello
There are a number of things it could be.
It is most likely a poor or moist physical connection in the pit in the street or corrosion etc in the cabinet outside your house. This requires a Telstra technician.
It also could be your modem.
It would be useful to eliminate your computer by testing with another computer and a different ethernet cable.
Below is a link to a self help page
https://www.telstra.com.au/webforms/cares/
I think you will need a Telstra tech to attend, and suggest he brings another modem to verify.
It is curious re the time frame of the issues. I don't have an answer for that