Hmmm, bike shops?
They are mainly roadie and MTB style places, there was a shop called (we think) Adventure Cycles in Aukland city centre which was a bit more free range, but most touring bikes came with their owners from overseas; this info is from 3 years ago and may be subject to change.
If you are planning to wwoof start north and head south, the one place I would avoid is Dargaville, the kumara capital of NZ, you'll easily get work picking the damn things, but it's like a graveyard, but with less atmosphere, decent sized town with three pubs, one had closed down, one shut at 7:30 even though it was half full of customers and the other had a carefully selected workforce who made us want to be anywhere else. Bay of Islands for fruit, East Cape for grapes.
Better still just go straight to the Mainland (South Island) it's much nicer, Blenheim and Renwick for wine and if you're any good with sheep they are everywhere.
I'll repeat the driver warning, in the cities we found them generally OK, but on the open road they were the worst I have ever mixed with. As to gang violence, every city has it, think Fratton, or even Leigh Park come to that. We never had any grief from Maoris at all, the only problems were with white skinned, Kiwi drivers.
If you can get hold of a couple of tiny books called "Pedallers' Paradise" (north and south island), by Nigel Rushton, they are very good.
