I have been on NBN for the weeks and have a 200gb plan which I thought crazy as l have never gone over the 20gb that l had and that was watching Netflix, Stan etc. Since going to the NBN I used 53gb in one day?? I rang and questioned it and got the same story that maybe a neighbour had hacked in. I turned my WiFi off for a day then back on but decided not to use it for two days. I only used my phone data. This way l could check if anyone had hacked in. No movement of data. Day four l watched a couple of Netflix and now 73gb data used. NBN is using way more than I have ever used and 200gb is not going to cut it. This is the biggest money grab ever. I'm looking at canceling it all together and sticking with my mobile.
Netflix automatically adjust the data rate to suit the connection. If NBN is faster than your previous connection you will use more data unless you manually limit the data rate in the netflix settings. A uHD video can use over 7GB per hour.
What device are you using for watching Netflix? If you are using an android or pc you can check usage in the devices settings. There are also traffic monitors in the modem located in the services settings.