I have been complaining about the excessive charges I have recieved since joining Optus and ive always been told its! Ive never believed it because when I was with Telstra I used to only use 8G of data a month, and now im on a 20G data plan with Optus I use it in 3 weeks. I have the same phone and same habbits! Doesnt make sense.
Recently I returned from 2 weeks in Indonesia (had my Sim out of my phone the whole time) and on my return I checked my data usage and ive been charged for 1-2 G data every day I was away!
Optus you guys are a bunch of criminals and ive already got fairtrading and the ombudsman involved. Cant wait to hear what you have to say about this one!!
I would think that if you were overseas with the sim card out of the phone it should be easy to prove or disprove that wasn't you as it would have been roaming data I would think. I would be having a word with support on the live chat link below and see what they have to say.
As for constantly using more data with Optus do you have a data usage meter on your phone like the one in the picture below? If so you can use that to check you are not being charged for data you are not using. I assume your account is a standalone and your not data pooling with another account?