It appears that after disconnecting the chromecast, the daily data consumption from Dec 5 - 10 has dropped and showed a usage between 6MB to 47MB.
I can see that you spoke to our Technical Team last night as well to which it was noted that you will be checking which specific devices has high utilization to try and isolate them.
If you have further questions or concerns, do let us know.
I'm just a random TPG customer who saw your post. I've been with TPG for nearly 7 years. TPG doesn't do what you imply. Although solving it can be frustrating, communication with the staff is key. If need be ask the staff member to slow down. Ask them to repeat in their own words what you said. Ask them to read the job notes to you. It will take time to nut it out.
In your home is every computer, and phones, all devices switched off? Samsung phones & tablets are crazy at doing stuff when you think it is not. I think to start with what you have got at your home that could be using data. I have noticed Chromecast keeps downloading whatever is in the list after I turn off the tv. Make sure Chromecast is off. Check Samsung devices.