I'm on a postpay plan with 2 Degrees, regarding data I get 100mb/month.

Occassionally I go over the 100mb, I get a txt from 2 Degrees when I have 10mb left advising me to add on another datapack, which I do - all good.

But then next month (or the next month I get to 90mb data usage) I get another txt from 2 Degrees advising me to get another datapack, which I do again. Nothing is said about the 100mb datapack that has auto renewed, so if I get another datapack I now have 10mb of my auto renew datapack, 100mb new datapack and 100mb of my postpay plan data.
I also note that as soon as I add a datapack it starts getting used, usually when I have 10mb data left.

I have spoken to and emailed customer support, they have advised me this is the way the system has been set up & it can't be changed, and to cancel the datapack as soon as I purchase it so it won't auto renew. They also advised me to monitor my usage and when I get to the end of my data to renew then, rather than when I have 10mb left!

I wonder if anyone here (or from 2 Degrees) could answer me a few questions.

1. why do I get txts from 2 Degrees saying I am running out of data when I am not?
2. why do the data packs auto renew - I am on a plan because I don't usually use more data than my plan!
3. why are the add on data packs used before my postpay data?

I (and my family) have been with 2 Degrees for a year now and generally everything has been great, but this is reaally bugging me. It brings up visions of Theresa Gattung and marketing obfuscation!