I cant thank you enough for your fantastic advice - this will really help me to fine tune what I want to do. The only reason Auckland is important and a base of sorts is because my sister lives there...
You've mentioned a few things that are not listed on any of the websites or blogs that I've read so thanks a ton for taking the effort to post your feedback/suggestions!
thanks so much groverkiwi..Ill check out the camper van options, too!
I agree with most of what's already been posted. I spent 6 mo on the south island and never ran out of things to do. My advice would be to spend most of this trip exploring the south island and save the north island for the next trip...since your sister lives in Auck you can go back, right? You could fly into Auck, visit family, fly to ChCh and begin your journey...drive the south island, do the ferry crossing as suggested and drive back to Auck...make a list of things you missed this trip and save it for you itinerary for the next trip, say your goodbys...
Get a car, get a car, get a car and yes the "I-Site" centres are in every town, these are your tourist info centres. Use them! Yes, Yes, Yes, the map can be very deceptive. What looks like a quick short drive can easily take half a day. General rule of thumb, double your time estimates for things to do and time to get there. NZ is such a spectacular place you won't want to feel rushed to get to the next destination.
And yes, the people are fantastic. If you would really like an adventure, stop and ask directions from a stranger - to your next destination. Not only will you get the information you need, you'll also get 3 or 4 or 5 suggestions for other stops along the way...places you may not have been aware of...this is great exploring. Kiwi's love showing off their country.
You and the kids will have a blast.