I used 100GB yesterday not doing much of anything so it is possible. Without any information on what services you have I can only suggest you check any backup services you may have tuned on. If this is mobile broadband maybe change the password. If you give us a bit more info we might be able to make some better suggestions.
Do you have an Android Phone? It should have its own local record of what data you have used over the past few days. Also what app used it. You can also install the Optus App and that will give a bit more information on what Optus think downloaded the data (youtube, etc.)
Same here. Never used much data but for the last two month it seems as soon as I hotspot my iMac to my iPhone I will use over 10GB in an hour. No streaming is active. Usage just says hotspot data usage. Only thing I am doing is sending a few emails.
My first idea was syncing of some sort. But this only happens when I use my phone as a hotspot but not when I use WiFi or use the phone on 4G without using hotspot. Also the Mac network monitoring tool will not show any app using the data but only records the data as an total in the upload. This should mean that Dropbox, iCloud, and time machine are not causing this excessive upload as they show up individually. I had Apple technical support remoting in and they could not explain. They get this investigated and I will report once resolved.