Yeah I am pretty sure that is the case but is there a way to find out how much data I am using.
Is there some sort of magic switch that can be flicked for it to be measured or other alternatives.
There's third party softwares that can measure usage such as netlimiter, however those require to be installed on each and every computer, (phones and tablets are excluded as they aren't supported).
Other than that and for a more centralised data logging, third party firmwares for routers (if supported) as well as having a computer doing all the data accounting is one other possible way.
JLo is right mate.
I do remember way back when I was using ADSL2+ with unlimited data under TPG. I was curious if how much data I was using every month to make sure that I utilized the package that I'm paying for. I've asked if where can I see this information and was told that it will not show up if I'm under the unlimited data plan, but was offered to send the information that I seek via email.
You can call or Email TPG to ask for those information and their support team will provide you what you want.
Your modem will probably have a "Statistics" page where you can read the amount of data sent and received.
On the HG659, it is on Maintain > Statistics. In addition it may also display (perhaps on another page) how long it has been up.