OP, December is the "high season" for traveling to/from New Zealand, what with it being summer and schools on holiday. I look at UA loads pretty frequently. That time of year, chances aren't very good to non-rev, at least on United (via Sydney). My wife and I have been to both NZ islands, but we chose to travel in the shoulder seasons (April and September). Loads this month are good as well to SYD.

You really need to get a guidebook or do online research to figure out what to see and do in NZ. You really have limitless options depending on your budget (surf, ski, bungee, fish, wine taste, hike, etc.). Lonely Planet just came out this month with a new edition of their NZ guidebook. It is an excellent book.