Does your phone have a breakdown of data usage under settings / data usage? Possibly the My Optus app under Data Insights might give you an idea as well if you have the app installed on your phone.
What phone are you using? As @Yeldarb says, first thing is to find out what is using the data (check your phone settings for app data usage too).
Biggest data users are usually TV streaming or Apple Sync. Either of those apply here?