Hertz doesn't mention any kind of restrictions on renting a car in New Zealand on the North Island (i.e. Auckland) and dropping it off on the South Island (i.e. Queenstown or Christchurch). Several well-known travel websites however mention this isn't generally allowed by any of the rental car companies in New Zealand.

Has anybody ever done this type of rental with Hertz before?

These same websites however have contradictory information saying that some rental companies with offer a discount to "move cars" around the island. They do not specify if this is between the two islands or within the island (such as between the major cities and the ferry ports). I'm researching for an April 2011 visit if it is better to rent in AKL, drop the car off before getting on the ferry, picking up a new car after hopping off the ferry, and then returning in either ZQN or CHC. Or should I pick up at AKL, take the car on the ferry, and drop it in either ZQN or CHC.