How do you prove a negative? Its tough.
Did you get the SMS warnings at 50%, 85% and 100%? (and any auto topups?)
What does your phone say about data usage over the last month? (If possible set a local warning and limit amount (14Gb?) on the phone settings - android and windows)
On an iPhone reset the mobile data counter each month.
Do you watch Netflix etc. and stream a lot of data or does Apple sync a lot to your phone?
IME Optus doesn't really negotiate on data usage. They can tell you broadly wahat the data was used on (eg. Netflix) and exactly what and when data was used on your account (request the detailed report). However from their point of view if the data adds up to more than 15Gb then that's what they bill.