The ancient penguin was much larger than Happy Feet, the emperor penguin who came ashore in New Zealand.

The fossilised remains of an ancient, human-sized penguin found in Canterbury suggest penguins once waddled the earth with dinosaurs.

A fossilised leg bone found in the Waipara River belonged to a penguin that stood 1.5 metres tall, roughly the size of a small adult.

It would have swam in the waters of Zealandia, the sub-continent which broke away from Gondwanaland.

CHRIS GASKIN/UNIVERSITY OF OTAGO The previously discovered Waimanu genus resembles a shag.

It was dated to 61 million years ago, making it among the oldest penguin fossils ever discovered.

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Research based on the specimen, just published in The Science of Nature, came to an "unexpected" conclusion – penguins are older than originally thought and likely existed alongside dinosaurs.

The conclusion came from comparison to another ancient penguin species, also discovered in Waipara.

The Waimanu lived during the same period but was smaller than the more recently discovered specimen and looked more like a modern day seabird, such as a shag.

Because the two penguins were vastly different, their evolutionary process would have begun millions of years earlier, during the Cretaceous period.

"We believed up until this specimen was discovered there was very little variation among these Paleocene penguins," said Dr Paul Scofield of Canterbury Museum, who co-authored the research.

"We're now starting to understand that shortly after the extinction of the dinosaurs there were in fact two quite different groups of penguin."

The recent fossil was discovered by Leigh Love, an amateur fossil hunter.

Because it was only one small part of a single penguin, the species had not been named.

Based on the fossil, researchers estimated it was much larger than modern penguins, but resembled them in many other ways, including the distinctive waddle other penguins of the period did not have.

"It's one of a small group of truly giant penguins," Scofield said.

"This particular specimen looks very, very similar to modern penguins."

The discovery was significant and would likely be used as an "anchor point" to determine how species evolved.

It is likely other penguin species existed during the same period that have not yet been discovered.

"We've really only scratched the surface of what was around at this time."

The Waipara River is one of the few areas in the world where the fossilised remains of ancient seabirds are found.