Travelpunk.com!!! You and your kids can concider yourselves part of the family here!!!!
I ventured through Melbourne and Sydney and spent some time up the central coast of Australia, but no tiume where your going to be. I found some great links for you destinations though, I hope they will help ya out some. We do have members from OZ and some that are trek'n through the area as we speak, so I hope they will pop in and add their 2 cents.
Anyways, here is my contribution to you preperation...
A Trip Through Middle Earth
Might be a great Idea for your kids, check this site that will help you plan some really cool "Lord of The Rings" stops on your trip in NZ
Tourism section for the city of Christchurch
Looks to be a nice place to stop when your in NZ, some ideas of things to do are on this link
City of Wellington recreation page
HAs some great ideas for things to do around the capital city...try it out!
Well thats what I got for ya...