The cormorants only take up a small area of forest. The main reason it is an Important Bird Area is that there are so many cormorants, gulls and terns breeding. It seems like a lot of birds, but the nearest colonies are in Hamilton and very far east, its not like they are destroying the forest all along the shore.
And keep in mind that the spit is completely artificial and very new - the cormorants have taken over forest that didn't even exist 30-40 years ago.