Shouldn't be an issue as Optus have been scammed not you. I'd just contact them in business hours and ask how to proceed. Optus don't appear to have any dedicated department for this issue so hopefully you find someone who can action a resolution for you.
Ironically the details given by the scammer might differ from your's so you won't be allowed to discus the specifics as you can't properly identify yourself.
If you do get stymied, I'd suggest writting a formal letter to the complaints department notifying them of the issue and refuting the bill. That puts any other processes on hold while you work it out.