This morning I've been reading through the document entitled "2degrees Value Pack Terms & Conditions" to try and clarify my understanding of how these things are supposed to bill, and there are some interesting things lurking in the detail. In particular:

 - Prepaid customers will have their data pack automatically renew upon expiry of that data pack, whereas account customers will have it renew upon the commencement of their next billing cycle even if they purchased the data pack part way through a billing cycle.
 - Prepaid customers will have their data pack automatically renew when the data quota is almost entirely consumed, but this does not apply to account customers.
 - If a Mobile Broadband pack (I think this refers to "in canopy" packs) runs out of its data quota and, for whatever reason, does not auto-renew then data will be charged at 10c per megabyte instead of the "prevailing rate" until the data pack reaches its expiry date. 
 - A Value Pack (my interpretation is that this covers both National and in canopy packs) will always be be consumed  before any data bundled with your monthly billed plan.
 - If a prepaid user has insufficient credit for a data pack to auto-renew then the auto-renewal will still occur upon paying a sufficient amount onto their prepaid account, provided that this is done within 14 days of the data pack's expiry date.

If you're a 2degrees data user then I would encourage you to download these terms and conditions and read them if you haven't already. I have a strong feeling that a lot of this detail is probably not widely understood by 2degrees' customer base.