What would be a probable explanation for why there are many endemic plants and birds in new zealand but no native animals

New Zealand islands are formed by volcanic activity, so they are not as old as the closest continents so that they could have some animals that evolved from the prehistoric species (especially the mammals). So when the islands formed, they were far away from the closest continent, Australia, and only the birds and the plants (through their seeds in the winds) managed to get in here and develop unique characteristics.

Magma is outside the volcano  and lava is inside the volcano. 

Answer// dinosaurs emerged during mesozoic era;