There are so many ways one can tour New Zealand, but I kinda like the slow and easy drive-on-your-own way. If you wish to self-drive, here's what you need to do in order to prepare:
1. Selecting the time frame.
The key words are "DO NOT RUSH". Spending 1 night or 2 in any ONE place will be inadequate. If you have 2 weeks (14 days), for example, I would advice you no more than 4 places to truly enjoy your holiday. Rushing through and seeing a lot has only 1 benefit and that is to tell everyone that you've been there and done that, but really, you would not have enjoyed yourself.
2. Selecting your tour itenarary.
What kind of holiday do you want? Adventure? Sceneries? all of the above?
There is simply so many nice places to go and so many things to do in New Zealand. Be specific in your plans. The key question is how much time do you have? See point#1 above to get a good feel. In general, the North Island is more "commercial" and the South Island is more scenic (my opinion anyway). What I did was to use an online tour company specializing in NZ, and then used that as my guideline to select my destinations and number of days per stop. The online tour should be able to cater for your needs. This I find was the easiest and most assuring because it means there is a definite place to stay AND it caters to my own needs.
3. Selecting your flight.
The flight to NZ is not near unless you're from that part of the world obviously. For me, I had to fly through Singapore, Sydney, and finally to Christchurch. Totaling almost 12 hours.
4. Your luggage
Other than your normal travel stuff (clothes, toiletries) and depending on the season, advisable items include moisturizers, lip balms, sun screen, rain coat (and winter clothes if in winter), swim suits, first aid kits.
Bring vitamin C, laxatives, paracetamol, band aid, (not likely you will have any food poisoning though), antihistamines if you have sinus problems.
5. Your Money
Bring enough cash. For the adventurous type tours, you can spend as much as NZD100-200 per 'adventure'. If you dont feel comfortable carrying all that cash around, you can also use your credit card, but make sure you inform your cc provider so that they dont "freeze" the account seeing that a transaction is not coming from your home country. How much will you need to budget for? This is hard to say and depends entirely on where you stay, what you do, and what you eat. However, if you are like me and able to live in moderately comfortable, easy diet, and fiendish adventure needs, and loving breathtaking sceneries then I suggest you budget about NZD200-250 per person per day. (motels:100,tour:100,food:50) Of course, this is only a guideline, remembering that motels can fit many people a time. However, again, make sure credit card is on hand.
6. Self Driving
If you're gonna drive in New Zealand, refer to the websites for the regulations about licencing. For all intents and purposes, in general, you should get an International Drivers licence (licences which are NOT in English will NOT do).
Driving anywhere at anytime in NZ is easy as pie. Just keep to the limits (City:50kmh, "highways":100kmh) and you'll be fine.
7. Insurance
Travel insurance covering your luggage, hospitalization, accidents etc are a MUST. See your agent to get travel cover. They are not expensive these days, and takes a lot of worry and headache away in event you do need some hospital care overseas.
7. Car Rentals
As long as you have your licence in order, you can handle the rentals when you arrive at the airport or go online and make your car reservations. I suggest online just so that you can arrange and be sure of the car you'll be getting.
