New Zealand was once home to a bizarre, king-sized bat that walked on four limbs and hunted its prey amid the undergrowth of ancient rainforests.

The scientists who discovered the new species, much larger than today's average bat, say the mystacina miocenalis could force a rethink on what we know about bats in this part of the world.

Their exciting find also shows, for the first time, that bats of the quirky mystacina species, which include our native greater and lesser short-tailed bats, have hung out in our forests for more than 16 million years.

The big bat was discovered from fossils in Central Otago, in sediment left over from a vast, prehistoric body of water known as Lake Manuherikia, where scientists have already found New Zealand's oldest frogs, lizards and land birds, and our only crocodiles and terrestrial turtles.

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Its discoverers, from the University of New South Wales, University of Otago and South Australia's Flinders University, have detailed the bat in a study published today in the scientific journal PLOS One.

They say the extinct bat fed on fruit, nectar, pollen, insects and spiders, and had structures in its limbs that were specialised for walking. At around 40g it would have been about three times bigger than the average bat today.

The mystacina bats are known as "burrowing" bats because they forage on the ground, scuttling on their wrists and backward-facing feet, while keeping their wings tightly furled.

But until now scientists had only been able to guess they had been here for millions of years. "Confirming that we've had this mystacina species for such a long time is great, and it adds to the oddball things New Zealand already has in its modern-day biota, like tuatara," said study co-author Dr Daphne Lee, of Otago

The age of the newfound species could change our understanding of when these peculiar walking bats first crossed the Ditch.

Lead author Associate Professor Suzanne Hand said understanding the links between different land masses and their resident bats was vital to gauging biosecurity threats and conservation priorities for fragile island ecosystems.