The best time of the year to visit New Zealand is December to April over the summer months.
While New Zealand is a small country with a population of only around 4 million people, the roads are mostly two lanes and so travelling around by car is a lot slower than in the US, the speed limit is also less at 100Km per hour, and generally between main cities you are averaging 80 kms per hour.
So the biggest mistake people make when visiting is thinking they can cover both the North and South Island in a two - three week visit, the reality is this is very difficult and would mean spending most of your time seated in a moving vehicle and concentrating a lot while driving due to the fact the roads are narrow and wind around varying terrain.
So if you are planning on visiting for just a few weeks, i would advise picking one island and spending all your time there, or go for a longer period of time.
The North island is great for beaches, wineries, getting out on the water and fishing, diving, swimming.
While the South Island is well known for the Southern Alps, glaciers, fiords, rivers, fly fishing, hunting, skiing, the Sauvignon Blanc and Pinot Noir vineyards, whale watching, spectacular vistas.
So it's best to pick one island and focus your time time there.
If you are visiting the North Island you will start in Auckland the main City in New Zealand.
This is like any big city, so you should limit your time spent here, and get out of town or at least get out on the water, there are many islands in the Auckland area which include Waiheke, and Great Barrier Island all can be reached by ferry service. You can catch a fish in the harbour and safely eat it.
You have several choices you either head north to the Matakana, Orewa Beach ( the home of Waiwera bottled Water) The Bay of Islands, Cape Reinga, visiting the Kauri Tree forests and the Kai Iwi Lakes and 90 mile beach or south to the East Coast and places like Tauranga, Coromandel, Rotorua, Lake Taupo, Napier ( probably not much point going further south than Tongariro National Park.
If you are going to visit the South Island, it's best to fly straight to Queenstown and start from there, it's worth basing yourself in Queenstown for 3 - 4 days, as there are many things to visit nearby such as : The Milford Sounds ( which can be reached by a long all day bus ride , or for the adventurous ones by a short helicopter ride which includes landing in the snow for a short play) , Arrowtown, many great wineries, lots of adrenaline activities like Bungy Jumping, Wanaka, Franz Joseph Glacier and the list goes on.
We have New Zealand Experts on staff who can help you with planning an adventure to the Southern Hemisphere by air, land and many great cruise options for that region.