In case I forget about stuff I've found:
Searching for bus passes that I could buy at the start so I won't have to worry about it later when I might, potentially, be penniless and whoring on the streets of Auckland.
Found InterCity Bus Passes/Flexipass seems to be the best option. They do it buy number of hours travelling, the biggest being 60 hours at $443, which is £210, and then with my YHA membership discount of 15%, that makes it roughly £180. That's £3/hour of bus travel. Bargain!
InterCity also do short trips, even day trips for what seems like not very much at all. Oh, and the ferry crossing is included so I won't get tripped up trying to get to the south island without enough cash.
Hmm, what else. I was going to go on a tour at the beginning of the trip, make some friends and see the country that way but it would be way too expensive.Still considering doing one of those KiwiExperience things, but from looking at the prices of InterCity compared to these, it's really way too much. Accomodation is not included, so you might as well get a cheaper bus pass and use a guidebook.
Got another year on my mobile contract that I can't get out of and I won't be able to use while I'm over there as the sim will be wrong. Have to find someone to give my sim to so they can use out the rest of my contract. Otherwise, it'll be nearly £150 down the drain.
I knew that organising all this would be troublesome, but there's always something else hat you didn't think about. The big stuff, like flights and visas have been easier than I expected, and were hardly any trouble at all, but bloody hell, this mobile sim thing really has me thrown.