Fossils of the largest parrot ever recorded have been found in New Zealand. Estimated to have weighed about 7kg (1.1st), it would have been more than twice as heavy as the kākāpo, previously the largest known parrot.

Palaeontologists have named the new species Heracles inexpectatus to reflect its unusual size and strength and the unexpected nature of the discovery.

Prof Trevor Worthy of Flinders University in Australia, the lead author of the research published in the journal Biology Letters, said: “Once we decided it was something new and interesting, the challenge was to figure out what family it was from.

“Because no giant parrots have been found previously, parrots were not on our radar – thus it took some time to differentiate all other birds essentially from parrots to conclude that the unique suite of characters was definitive of a parrot.”

Paul Scofield, a senior curator of natural history at Canterbury Museum, said the fossil had been excavated in 2008, and initially the team had thought the bones were part of a giant eagle.

The bones, which will go on display at an exhibition in November, were found in a fossil deposit from the early Miocene epoch, about 19m years ago, near Saint Bathans in Central Otago, New Zealand.

Although the area is now very cold and known for its skiing, Scofield said the climate at the time meant the parrot would have lived near a giant lake in a diverse subtropical forest.

“Back then, it was a subtropical environment quite similar to northern New South Wales, and even had similar vegetation,” Scofield said.

He said the parrot’s weight meant there was a possibility it was flightless. Although the bird’s diet is unknown, Scofield noted most parrots today are vegetarian.

“But as animals get larger, they become predatory,” he said. “It was so large it would have required a considerable amount of calories per day.”

Scofield said there are other examples of omnivorous New Zealand parrots that eat seabirds.

Ecosystems on islands frequently produce large, unusual, often flightless birds – for example, the kiwi of New Zealand and the dodo of Mauritius. New Zealand has produced a particularly large number of such species due to its large size, complex ecosystem and lack of predators.

Daniel Field, an avian palaeontologist at the University of Cambridge, who was not involved in the research, said: “It has to do with arriving in a place where there are no terrestrial predators.

“Unfortunately, that can lead to animals becoming susceptible to extinction in the event that terrestrial predators are introduced or evolved. It’s a tantalising finding and we’d love to know more about these extinct birds. I hope they find more remains in the future.”

Worthy’s team is planning to study songbirds from the same period. “There remain several groups of birds from the Saint Bathans Fauna to describe and reveal to the world, notably what were the songbirds we had back then – do they help reveal the origins of the modern endemic songbirds, or do they show closer relationships to things from Australia?” he said.