New Zealand is a good country for hitchhiking. There are a lot of friendly people. By hitching you´ll be involved a lot in the local life. You´ll have invitations to parties, homes.
Border crossing: Might not be that pleasant if you represent any other race than white. If you don't believe, look at whom are searched at the airport.
Cars drive on the left side, so be careful if you're not experienced with that.
There are only highways around the big cities, Auckland, Christchurch and Wellington, which makes it easy to hitchhike when it's not raining. People working at gas stations seem to be pretty friendly so this is your best bet when it's raining.
On South Island you will often be picked up by travellers.
newzealand.com has a useful map service.