Prehistoric moa could actually fly but lost the ability after the extinction of dinosaurs made them "fat and lazy" on land, a new study suggests.

The discovery debunks global theories about the age of flightless birds and suggests the moa's ancestor flew to New Zealand from Antarctica, according to Kiwi scientists.

Science had long thought ratites – Earth's largest flightless birds, which include moa, ostriches, and emus – descended from a common flightless ancestor.

But DNA analysis of moa bones by David Penny, Gillian Gibb and Elizabeth Crimp from Massey University revealed that the moa was closely related to the tinamou – a small flighted bird still found in South America.

When dinosaurs died 65 million years ago, some birds – like the tinamou – became land-dwellers, which allowed them to put on more weight, Ms Gibb said.

They eventually became too heavy to fly and simply gave up, evolving into moa in New Zealand as well as ostriches, emus and cassowaries in Australia.

The kiwi was also thought to be a descendent of the cassowary, she said.

"We've known for about 15 years that the kiwi possibly flew to New Zealand, escaping from Australia, but no-one realised the moa may have too.

"This shows we [New Zealand] are not an isolated, biodiverse, hot-spot in the South Pacific where everything has been here since day 1.

"There's a little bit more to the story than that."

The discovery would force a re-think of the theory that all flightless birds were established before the separation of continents 80 million years ago, she said.

Professor Penny said it was likely the tinamou flew, or was blown, to New Zealand from Antarctica before it froze over.

The team was led by former Massey student Matt Phillips, who now works at the Australian National University in Canberra. Its findings were published in this month's edition of Systematic Biology, an international science journal.

Te Papa fossil curator Alan Tennyson said museum staff would probably start educating the public on this new chapter in the moa's history.

"This [research] is important on a global scale. It has implications for anyone wanting to study the development of continents. It will be picked up all around the world."

It was always good to see Kiwis leading the way with ground-breaking studies such as this one.