In February I signed up for a new NBN plan. I specifically said that I did not want fetch as an add-on as I was sick and tired of being charged for this on my cable plan despite not having a fetch box. My plan was confirmed in writing. Then when I received my monthly bill I was charged an additional $10 for Fetch. Not for the first time I find myself begging Optus - stop charging me for something that I do not want, do not have, and did not sign up for!
Check further down in the bill. You may have a -$10 credit to counter the charge (thus making it free)
What NBN plan did you sign up for? $70? This issue sounds like some legacy entry that has been put in separately and just applied to your overall account. Unfortunately only Optus can fix it and you'll need to talk directly to someone.
As an alternative write a letter to the complaints address. These are considered official and get a complaint number and hopefully Optus will resolve it without any further input from you.
Customer Relations Group, PO Box 306 Salisbury South 5106.