Here are some thoughts about visiting New Zealand.
If you want to visit when the weather is warm, and relatively pleasant then try late January to mid March. Most New Zealanders take their annual vacations between Christmas and Mid January, so you avoid the Christmas-New Year peak season. But as there are several anniversary holidays towards the end of January, different parts of the country have an extra day off on different weekends. Also, Waitangi day, on 6 February, is a national holiday - visiting the treaty grounds at Waitangi on this day is likely to be an interesting experience, as it has become a traditional occasion for protest. February is usually the warmest month with a lot of sunny days. The evenings are long as daylight saving is in effect.
If you like snow sports, then July to September are generally good times to visit as the ski fields are open for skiing. However, it is also a cold and wet time of the year for the rest of the country. This is also the time when bad weather tends to make travel more difficult, with some roads closed by ice and snow. Rough seas occasionally force the Cook Strait ferries to stop sailing at this time of year too.
Bring a rain coat - you will probably need it - Milford sound is spectacular on a sunny day and completely different but just as spectacular when it is misty and rainy. It really needs to be seen on both types of days to do it justice.
To see the thermal wonderland visit Rotorua. (Every now and again a new one erupts around town .)
North Island Cities
While Auckland and Wellington are the two main cities in the North Island and a trip between them can be done in a day by bus or train, most cities and towns cater for the tourist to some extent. Other cities to see are :
Napier - an Art Deco city rebuilt in the mid 1930's after being mostly destroyed by an earthquake in 1931.
New Plymouth - Womad music and dance festival, the windwand - a sculpture with a red light on top that moves with the wind. The city makes a good base to explore Taranaki and Mount Egmont National Park.
Queenstown, lake, steamship, jetboats, gondola, casino, golf course, 3 skifields, bungy jump ... adventure tourism capital of the world. With day long bus trips to Milford Sound and an international airport - for Australians.