Advise your Bank that you have ceased authorisation for the Direct Debit (DD) to be processed. This may or may not work but send a letter to Optus re the removal of your authority and send a copy of that to your bank with a covering letter. You if a member of CHOICE can avail yourself of CHOICE’s Help Service to get templates of letters to use and who it may be possible to contact:
There is your State’s Office of Fair Trading that may be able to help with getting Optus to stop.
There are a number of posts and topics on this site regarding automatic DDs. See the following as they may have some steps that may be useful in your circumstances.
I would also contact Australian Financial Complaints Authority (AFCA) about your issue if the Bank fails to stop the DDs occurring.
"The Australian Financial Complaints Authority (AFCA) can consider a complaint you have about a banking deposit or payment issue with your financial firm.
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Overview
Banking deposits and payments include:
deposits to current accounts and savings accounts
internet and telephone banking, including mistaken internet payments
payments, such as direct transfers or direct debits, electronic banking and foreign currency transfers" [my highlighting added]
