Whilst the New Guinea harpy eagle is frequently seen and heard, very rarely have individuals been observed at close range for extended periods of time (7). It is an accomplished predator, known to feed primarily on ground- and tree-dwelling mammals (2). This includes possums, wallabies, tree-kangaroos, giant rats, and even young dogs and pigs (2), but it will also prey on birds and reptiles (2), such as snakes and large monitor lizards (7). They hunt by day, perched in the forest canopy, searching the ground and surrounding trees for prey. Like other birds of prey, it is likely to have acute eyesight (5), and its facial ruff is though to assist in the detection of prey by sound (2). Once spotted, the harpy eagle swoops down from its perch and seizes the prey in its feet, and will run and leap along the ground in pursuit if necessary (2). Hiding among foliage or in holes is not always sufficient protection from this committed hunter; the New Guinea harpy eagle will strike at foliage with its wings, or tear at it with its feet, to flush the prey hidden within, and it can use its powerful feet to extract prey from holes (2).
The New Guinea harpy eagle is a solitary breeder, and constructs large stick nests high up in a tree (2). It apparently breeds from the wet season through to the dry season; a nest was seen in use from April to May (2), and it is thought that this bird may not breed every year (6). In captivity, the New Guinea harpy eagle has lived for more than 30 years (2).