The bird is thought to have stood at around 5ft10in tall and weigh 223lbs.
In contrast, the largest penguin now is an emperor penguin, which stands at around 4ft tall.
The fossilised partial skeleton is estimated to be at least 55 million years old.
The bird has been named Kumimanu biceae, from kumi, which means a large mythological monster in the Maori language. See more in the video above.