Cars

New Zealand has a great number of companies which rent vehicles of varying types.  Most international rental companies, such as Hertz, Avis, Thrifty, Hertz and Budget are represented in most towns. And there are many smaller NZ owned companies, like Nationwide, Ace, and Rent-a-Dent which are also good.

For the backpacker market New Zealand offers some of the best car and campervan rental deals with much better prices than the corporates, Bargain Rental Cars offers good value for your money, Apex Car Rentals is a good new zealand car rental company as is Go Rentals, offering very well priced quality vehicles for the budget conscious traveller.

There are  a good range of vehicles available, from the compact, to family sedans, as well as vans of various size and type.  Luxury cars  (BMW, Mercedes, etc) are also available, but in far lesser numbers. SUV's are common around Queenstown and Christchurch where 4WD is usefull getting to the Ski Fields.

Motorhomes, Campervans and RV's

There are many companies renting Camper Vans in New Zealand,  THL are the largest offering a variety of products from budget to family options, but there are many others, such as Iconic  or Walkabout Rentals who offer the only fleet 5 star luxury of European manufactered motorhomes. Their motorhomes are all guaranteed to be 18 months old or less. These motorhomes are also built on specialist chassis to give better road handling and ease of driving. 

These are very different from the typical RV as is common in the USA.  Most NZ rental Campers are built on a light truck chassis, rather that a specialist RV chassis.  Consequently they ride and handle like a small truck.  They also tend to be smaller than American RV's, although some larger ones are starting to appear on the roads now.

These are by no means inferior to the luxury operators and often offer comparable products at a lesser price. It is important here to check public feedback, reviews and their terms to ensure you compare apples with apples. Some highly recommendable '2nd tier' operators are Wendekreisen and Euro Campers.

Minibuses and People Movers

There are  a good range of vehicles available, from Minibuses to people movers. These vehicles are ideal for any type of group travel. From sports teams, clubs, schools, field trips, scenic tours or weekends away. Hireace offers great value for 10 to 13 seater Minibuses and 7 to 8 seater People Movers hire. 

Insurance

In New Zealand, by law, Collision Damage Waiver (CDW) and theft protection are included in all rates. The CDW deductible (responsibility of the client) ranges from U.S.$1200 to U.S.$2390, depending on the supplier and vehicle type. Theft protection deductible also ranges from U.S.$1570 to U.S.$2390.   You will be offered the opportunity to reduce the excess (CDW)  to something more managable, usually in the vicinity of NZ$200 or zero with some of the better companies, for a daily charge.  However, if you rent a car (not a truck or RV) you may not need this coverage, as Visa automatically provides coverage for the full amount of this deductable, provided that the driver of the vehicle at the time of the incident is authorized by the rental company and provided also that the rental company's reduced CDW deductible coverage was waived. 
Good company in New Zealand to arrange insurance to cover your vehicle security deposit up to $6000 this covers both motorhome rental and car hire in New Zealand - This is Comprehensive Insurance  - Instant online price and policy issue . Dont take the zero bond waiver insurance through rental companies as it becomes too expensive.

Note that American Express does provide this coverage in New Zealand for up to 31 days although most companies do not accept American Express, and Master Charge provides this coverage only for rentals of less than two weeks.  If you have an accident, most companies will take a credit card imprint for the excess of the insurance, and will only be charged if you have damaged the vehicle. If you purchase your own insurance, make sure the insurance covers glass as well, as a lot don't, it's extra. Glass insurance is important in NZ, as the roads have a lot of loose stones on them. Plus the glass cover also covers the windows if your car gets broken into, those little windows that theives break into are very expensive!

Also consider that many other drivers are not insured on New Zealands roads as insurance is not compulsary.

Third party Insurance is not compulsory in New Zealand,  and much has been made of this elsewhere in these forums.  While it is true that there is no compulsory 3rd party,  what they do have is a comprehensive Accident and Rehabilitation cover (ACC).  This is funded by levies on employers, and by other means.  Every person in New Zealand is covered, whether a resident or a visitor.  If you are involved in an accident of any type while in NZ , your complete care will be paid for by ACC.  They will also pay a lifetime allowance if you are perminently disabled.  Also,  if  (perish the thought) you are killed as a result of an accident, your body will be flown home, and any expenses incurred by all members of your family will be covered.  It is extremly comprehensive, and is mostly very generous.  What you give up in exchange for this is the right to sue for damages.  ACC may sue if they feel there has been culpable neglegence, but you will not see any of this.  This is a broad outline of the scheme,  and there are many why's and wherefor's but generally you can assume that if you are injured you will be looked after, and it won't cost you a cent.