First, lucky you. I'm heading home to NZ in February but only for two weeks and with my in-laws. I'll try to sneak in as much fishing as possible but I really want to do what you're doing.
Second, buy Trout Bum Diaries Vol 2 to whet your appetite. It's on my iPhone and I keep drooling over it.
The weather in December can be unsettled. (Mid summer is more like February.)
Even though I am from the North Island I would recommend biaising your time towards the South Island.
I think we'd need to know more about what other interests you'd like to explore while there. For example, are you a wine enthusiast? Do you like jumping off bridges with a rubber band tied to your legs?
In the North Island, you want to spend time around all the streams and tributaries of the Waikato area, you must fish the Mohaka, and the Tuki Tuki is worthwhile also.
You'll undoubtedly fly into Auckland. If you want to visit Northland (less fishing unless you want to chase Kingies with a saltwater rod) then do that first and head south from there, through the Waikato, across to Hawkes Bay (fishing the Mohaka on the way) to drink some of the best wine in the world and fish the Tuki Tuki. Then south, perhaps stopping in the Wairapa region (another great wine area).
Google for guides to get an idea of locations etc.
Here's a place I am going to visit on my trip which will give you a sense of the Mohaka:
http://www.poronui.co.nz/Home/Fly-Fishing.aspx
Here's a link for the Fish & Game access maps for Hawkes Bay fisheries:
http://www.nzfishing.com/AboutFishin...Bpamphlets.htm
(Well worth looking for the Fish & Game website)
Some random South Island:
http://www.owenriverlodge.co.nz/rivers.htm
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Kahawai are basically a type of sea trout. They come in and out of river mouths/estuaries with the tide. I fished the river mouth of the Ngaruroro river for kahawai as a kid. Great fun.
Here's the Fish and Game website:
http://www.fishandgame.org.nz/Site/R...Pamphlets.aspx
---------- Post added at 01:02 PM ---------- Previous post was at 12:55 PM ----------
Also, if you're interested in the local entomology and tying flies for the trip then buy this book:
TROUT STREAM INSECTS OF NEW ZEALAND - HOW TO IMITATE AND USE THEM
by Norman Marsh