Hi. I joined this forum just to give you advice on visiting NZ! lol! It is one of my favourite things to do. (Just so you know, I'm a Michigander who has been living in NZ for 8 years.)
First: glad you're going to Tiritiri. It is so wonderful. The number of visitors does not decrease your chances to seeing birds, as previously said. The birds are very used to people. Kaka do not live on Tiri but very occassionaly visit from Little Barrier Island--but in my 8 years of going to tiri at least twice a year, I've never seen a Kaka there). BTW, the boat ride to tiri from downtown auckland is about 1 hr 15 mins, not 30 mins as mentioned somewhere on this thread.
Kapiti is beautiful and has a different feel from tiri but I much prefer tiri.
As for rotorua, in some ways it is a big tourist trap, but a nice one! There are so many motels and souvenir shops, but I still love it. All the geothermal sites are really cool--my favourite is the boiling mud--I could watch that for hours. The maori cultural stuff is great too. You said you were going to go to Te Papa in Wellington instead. It is a great museum, but doesn't replace a visit to rotorua, so I guess you just have to decide which you'd like to see.
While in Wellington, go to Karori Wildlife Sanctuary
http://www.sanctuary.org.nz/
You'll most likely see kaka there.
I guess you've decided to skip Stewart Island. Too bad, but makes sense since you don't have much time here. I do recommend the Bravo Adventure Cruises--have been twice and thought it was great.
If you're going to be in Christchurch and you aren't going to be able to see a kiwi otherwise, go to Willowbank Wildlife Reserve. They have the best Kiwi House that I've seen in NZ--the kiwi aren't behind glass like at every other kiwi house--they are right there in front of you, you could reach out and touch them--you can hear them snuffling for food and smell them too. We were so close to them. I didn't like the small cages for the primates that I felt had no business being there, but the kiwi house was fantastic.
http://www.willowbank.co.nz/reserve.asp
Hmmm...What else....I'll post again when I think of something!