<P> The biodiversity of New Zealand, a large island nation located in the southwestern Pacific Ocean, is varied and distinctive accumulated over many millions of years as lineages evolved in the local circumstances . Recently a component has been introduced by humans . New Zealand's pre-human biodiversity exhibited high levels of species endemism, but has experienced episodes of biological turnover . Global extinction approximately 65 Ma ago, resulted in the loss of fauna such as dinosaurs, pterosaurs and marine reptiles e.g. mosasaurs, elasmosaurs and plesiosaurs . The ancient fauna is not well known, but at least one species of terrestrial mammal existed in New Zealand around 19 Ma ago . For at least several Ma before the arrival of human and commensal species, the islands had no terrestrial mammals except for bats and seals, the main component of the terrestrial fauna being insects and birds . New Zealand has developed a national Biodiversity Action Plan to address conservation of considerable numbers of threatened flora and fauna within New Zealand . </P> <P> The break - up of the supercontinent of Gondwana left the resulting continents and microcontinents with shared biological affinities . Zealandia (the continental crust from which New Zealand and New Caledonia later developed) began to move away from Antarctic Gondwana 85 Ma ago, the break being complete by 66 Ma ago . It has been moving northwards since then, changing both in relief and climate . About 23 million years ago New Zealand was mostly underwater . One estimate suggests just 18% of the present surface area remained above the water . However geological evidence does not rule out the possibility that it was entirely submerged, or at least restricted to small islands . Today about 93% of the Zealandian continent remains below the sea . Several elements of Gondwana biota are present in New Zealand today: predominantly plants, such as the podocarps and the southern beeches, but also a distinctive insect fauna, New Zealand's unusual frogs and the tuatara, as well as some of New Zealand's birds . It seems likely that some primitive mammals also were part of the original cargo . Whether or not any of these taxa are descendents of survivors of that ancient cargo remains unproven . Recent molecular evidence has shown that even the iconic Gondwanan plants the southern beeches (Nothofagus) arrived in New Zealand after separation of Zealandia from Gondwana . There is a high rate of interspecific and intraspecific hybridisation in New Zealand plants and animals . </P> <P> The two sources of New Zealand's biodiversity following separation from Gondwana have been speciation and air - or sea - borne immigration . Most of these immigrants have arrived from Australia, and have provided the majority of New Zealand's birds and bats as well as some plant species (carried on the wind or inside the guts of birds). Some of these immigrants arrived long enough ago that their affinities to their Australian ancestors are uncertain; for example, the affinities of the unusual short - tailed bats (Mystacinidae) were unknown until fossils from the Miocene were found in Australia . Cyanoramphus parakeets are thought to have originated in New Caledonia and have been successful at reaching many islands in the region . The link between the two island groups also includes affinities between skink and gecko families . </P> <Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table>

What is the main reason australia and new zealand have such unique biodiversity