Carry over data = 1 year expiry
Standard national data packs = 1 month expiry
Broadband zone data packs = 2 to 6 month expiry depending on pack.

The data expiring soonest is used first.

Example:
June i have 500mb of data carrying over to the next month
July i buy a 12gb 6 month pack
August i buy a 100mb national data pack.

If Inside a broadband zone: It will use the 12gb first since zone data is prioritized, then the 100mb pack.
If Outside a broadband zone: It will use up the 100mb pack first since its expiring soonest and zone data is unavailable.

Then the carry over, as thats expiring in 12 months from when i bought it.

edit: fixed as per below.

Unless something has changed recently the zone packs get used first (when in a zone).
then data packs, then carryover data.