3) Make a complaint to the ACCC. They won't help you specifically but hopefully enough complaints will see them look into Optus (again) for exposing their customers to yet another way to stip them of money that they can't control.
But the TIO (Telecommunications Industry Ombudsman) will help you specifically if Optus does not resolve the issue to your satisfaction.
The TIO is usually more concerned when Telcos don't provide what the contract says. Unfortunately the 1gb Auto Top up is unashamedly front and centre in all contracts and the CIS
True, but this is a billing issue. Optus have asserted that @alex6489 has exceeded his quota by ? GB, they have to be able to back that assertion up with evidence like providing session logs showing how they arrived at that extra usage. Then @alex6489 can compare that log to his usage pattern to determine if it was even possible that he used as much data as they claim when they claim. We are talking multiple GB on a mobile device here, so there has to be some clues in the log.
@alex6489 What type of device do you have? Do you have any auto updates enabled for the device OS or apps?