I changed my plan from ADSL to NBN in February. Since then I've been getting billed for both the ADSL and NBN. I've spent approximately 5 hours on the phone over the last three months to try and get this rectified - to which everytime the operator has said - oh yes - there is an issue - I'll rectify that now for you... to my amazement - low and behold - the next billing cycle - it is still there - along with a few additional fees 'for late payment' of a product that I technically don't even have.
Lets not mention the complete debacle - that was the connect of the NBN (where we were told that it was connected - yet the NBN Guys hadn't even been - so that took almost a month to get that right)
Why would I pay for a service that I upgraded from?
My suggestion is to make a complaint to the Telecommunications Industry Ombudsman (TIO) - https://www.tio.com.au/
You have given the people at Optus many opportunties to do the right thing. They have failed. It is time to make a complaint. It is a pity that nobody in management will be held personally responsible for their bad decisions, so the bad service and bad decisions will continue.