I enquired about this and 2degrees responded with their data usage policy, which I have reproduced here for anyone who may be curious.
According to our data usage policy, the following is the order in which the various data will be used from your services.
1) Mobile Broadband Zone Data Pack (Value Pack)
2) National Data Pack (Value Pack)
3) Plan Data - Allocated with the plan
4) Any Bonus Data - Allocated with the plan
5) Shared Data - if sharing data from other user
6) Standard Data Rates apply hereafter.
The reason why your data is being used from the Broadband Zone Data as it the MBB ZONE DATA PACK (VALUE PACK) which is the first to be used according to the hierarchy.
I guess like many things in life, timing is everything:-)