This is not so much a question as an answer. I recently incurred significant charges for using my iPad while overseas. I want to relate how this happened so others will not be caught. I used my iPad on a high speed broadband wifi in a private home (but with global roaming turned on). As my Optus contact explained, US 4G is faster than most home wifis. So when viewing HD video on an iPad, if the download speed from the wifi modem is not fast enough, the iPad switches automatically to 4G data. It can switch up and down between the two depending on the speed available and the user will not know. This results in significant charges for global roaming, even if you think you are only using high speed broadband. So beware. I want to thank Michael and his manager on the Optus help line for explaining this situation to me and assisting in dealing with the problems of a seriously high Optus mobile bill when I got home. Thanks Optus.
Sorry to hear about your bill. Unfortunately Apple still won't provide a simple way to limit 4G data on their phones and tablets. Android phones can set a local warning and cut off limit which would have helped stop your scenario.
Note that Optus are legally obligated to send data SMS usage warnings but these can (unhelpfully) take up to two days to reach you. If you didn't recieve these SMSs (at all) then its possible Optus are liable for the data charges.
FWIW all devices can sometimes switch from wifi to 4G without notice. I would generally recommend getting a local data SIM for travellers as there's really no reliable way to limit your exposure to what are really extortionate data charges.
Ipads cant receive SMS, so if its your only mobile device on Optus then you would have to rely on any email alerts (thats if its been setup)