New Zealand is a reasonably okay 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. An useful site to find lifts is backpackerscarpooling Cars drive on the left side, so be careful if you're not experienced with that.
Another awesome place to travel, WWOOF and hitch. Not as many long distance rides as in the Australian outback, but towns were quite close together. Amory Tarr had no troubles coming up and down the west coast (South Island), or in fact anywhere really. The only problem spot was in the concrete jungle that is auckland. It is illegal to even walk on any of the motorways, including the on-ramps ($200 instant fine). It is also a very, very big city to walk across. If you want to head south your best bet is to go to the bus station under the sky tower and get a ticket to Bombay junction (about $15) and when you get there, walk across the road to the on ramp which has plenty of space. Heading north, get a bus to Orewa (which is worth visiting on its own) and hitch from there.
On the South Island you will often be picked up by travelers, especially along the West Coast, where there isn't a lot of traffic.
If you are hitching between Greymouth and Wanaka you might have the fortune to score a ride with one of the DHL drivers aka the local posties. They are a couple of awesome guys who meet up in Fox Glacier to swap packages (and hitch hikers too if you are lucky!).
An easy way to catch your lift is using the website of backpackerscarpooling.com in New Zealand Here you'll find travellers sharing their transport. A great way to meet like-minded travellers going the same way as you.
Summers start in October/November but being a maritime climate the weather can be changeable. The weather in January - March is a little more settled.