eXDee: ^Probably because they don't have 900mhz 3G in that location, and the 2100mhz 3G is too weak so you're falling back to 900mhz 2G.
This is what happens in the majority of cities on 2degrees in marginal coverage areas except those later in the rollout, eg i think Dunedin onwards included 900 3G. There's probably a few sites nationwide though that have been upgraded, i'm sure the reps here can clarify. Auckland, Wellington and Christchurch were done early on in the rollout.

The same goes for Vodafone, except where they have done upgrades such as auckland, working on wellington atm and i believe christchurch too?

Telecom simply has 850mhz 3G everywhere and 2100 3G in some locations.

If you didn't know, low frequencies travel further. Please note this assumes you're using a phone that is fully compatible with 2degrees network - for example not some sub $500 telecom device that wouldn't support the 900mhz frequency anyway, only telecom's 850mhz.