During the middle Cretaceous, New Zealand and Australia were linked to Antarctica,
but not to each other. Antarctica was also linked to South America, thus making
migration of plants and animals possible between these areas. It was during this
time that anigiosperms arrived in New Zealand.
By 80 million years ago, New Zealand was no longer attaced to Antarctica. Proof
of this comes from the fact that marsupials moved from South America to Australia
during this time, but not to New Zealand.
That having been said, during early Cretaceous times, monotremes (platypus and echina)
migrated to Australia, but not to New Zealand. This probably happened because the
monotremes migrated through Eastern Antarctica (due to the warm weather of that
passage) to Australia but, once there, they were blocked by the Tasman Sea (separating
Australia from New Zealand)and simply went no furthur.
Until recently, it was thought that New Zealand was devoid of dinosaurs, as no sign of
any had ever been found. In 1986, an "amateur paleontologist", Joan Wiffen, found
two small bones from late Cretaceous. Although they don't tell us much about the
dinosaurs themselves, they do confirm their presence in New Zealand.