What would be a probable explation for why ther are many endemic plants and birds in new zealand but no native mamals

During 5e continental shift as you may already know Australia and New Zealand split apart right, but during this time most of the mammals that were on Australia and New Zealand were split, with Australia staying with the mammals, and New Zealand with the majority of the birds and reptiles and amphibians, that explain why Australia has more mammals than New Zealand, and why New Zealand has more birds,reptiles, and amphibians than Australia. Hopefully this helps