I was out in NZ last June and spent most of my time
not climbing on the North Island due to the difficulty of finding any info on reasonably close areas, so I gave up and headed to Castle Hill for 4 days by myself.
I stayed in Springfield town, which if you have no transport of your own is an easy place to get dropped off/picked up on the bus from/to ChCh.
I stayed here:
http://www.yha.co.nz/Hostels/South+Island+Hostels/Springfield/ and hitched into the boulders every morning and back every evening. Its about 25ish minutes drive (I think) to where the boulders are and extremely easy to get picked up, even with a pad!
Springfield also has a supermarket that sells tape, chalk and a wide variety of canned goods....all at extremely inflated prices...
Its obviously less viable for long periods of time as your paying for the hostel (They also rent pads...even though they're a little crappy), but if its a short enough stay and you dont have transport I'd rate it as a good option.
The Hostels really nice aswell! Lovely, massive fire every evening and there were some really nice guys up there fixing the railway tracks when I was there....the guy who runs it is a little odd though...