Tiritiri Matangi is a life changing experience: Almost what New Zealand's great bird song was like before the arrival of humans and pests. The native bush is only around 30 years old as the island was farmed for a long time after the arrival of humans. Great island.
Rangitoto and Motutapu is great for views of the city - and recently freed of pests aswell. How ever don't expect the bird song you would find at Tiritiri Matangi for a while yet.
My Recommendation Tip
Most enjoyable island for its bird life in the world!
It is also a very stunning island to walk throguh while listening and watching the birds.
New Zealands birds are known for their beautiful calls and variation of song, However they have been wiped out from most of mainland New Zealand due to the introduction of ground based mammals: e.g cats, rats, mice, stoats, rabits e.t.c.
This may sound amazing, but before Humans arrived in New Zealand There were no mammals exept for two species of bats: Allowing bird life to thrive and become very different to birds found elsewhere in the world.
Go and see it! You will love it!