Image copyright Canterbury Museum Image caption The enormous penguin, as reconstructed by Canterbury Museum

The remains of a giant penguin the size of a human have been discovered in New Zealand.

The fossilised bones are of an animal thought to have been about 1.6m (5ft 3in) tall, weighing up to 80kg (176lb).

It lived in the Paleocene Epoch, between 66 and 56 million years ago.

The animal, dubbed "monster penguin" by Canterbury Museum, adds to the list of now-extinct gigantic New Zealand fauna. Parrots, eagles, burrowing bats and the moa, a 3.6m-tall bird, also feature.

Why was the penguin gigantic?

"This is one of the largest penguin species ever found," Paul Scofield, the museum's senior curator, told the BBC. It was specific to the waters of the Southern Hemisphere, he added.

Penguins are thought to have become this big because large marine reptiles disappeared from the oceans, around the same time that dinosaurs disappeared.

"Then, for 30 million years, it was the time of the giant penguins," Mr Scofield said.

Today's largest species, the Emperor Penguin, grows to about 1.2m tall.

"We think that at the time, animals were evolving very rapidly," Mr Scofield explained. "Water temperatures around New Zealand were ideal back then, around 25C (77F) compared to the 8C we have now."

Image copyright Canterbury museum Image caption It's a small bit of bone - but a sizeable discovery

During the time of the giant penguin, New Zealand was still joined with Australia, which in turn is thought to have been connected to Antarctica.

The new species, Crossvallia waiparensis, resembles another prehistoric giant penguin, Crossvallia unienwillia, which was found at a site in Antarctica.

According to the researchers, the Crossvallia waiparensis penguin's feet probably played a bigger role in swimming than those of modern penguins.

It likely shared the waters around New Zealand with "giant turtles, corals and strange-looking sharks," Mr Scofield says.

Why isn't it around anymore?

It's not entirely clear why the giant penguins disappeared from the waters of the Southern Hemisphere.

The most commonly held theory is that it's simply due to growing competition with marine mammals.

"At the time giant penguins evolved, the large marine reptiles just had become extinct," Gerald Mayr, another author of the new study, told the BBC.

"In Antarctica and New Zealand, there were no large marine competitors until the arrival of toothed whales and pinnipeds (seals) many million years later."

The extinction of the giant penguins seems to correlate with the rise of these competitors, but the exact reasons for the disappearance of giant penguins are still being discussed, Mr Mayr cautions.

Where was it found?

The leg bones of the new species were discovered at a site in North Canterbury last year and have since been analysed by an international team.

"This site is pretty much unique," Mr Scofield said. "It's a river bed cutting into a cliff."

The place has been the site of fossil finds since the 1980s, and many of the discoveries - like this latest one - are made by dedicated amateur palaeontologists.