Answer:
Due to isolated before mammals evolve.
Step-by-step explanation:
There are many endemic plants and birds in New Zealand, but no native mammals because New Zealand is an island which isolated from Gondwana which is a supercontinent about 85 million years ago. Before mammals evolved, the New Zealand splits and go away from the Gondwana. So due to water only those mammals that can swim or fly colonized in New Zealand but not the land living mammals.