Female Haast's Eagles are believed to have weighed 10 to 15 kg (22 to 33 lb), and males 9 to 10 kg (20 to 22 lb). They had a relatively short wingspan of roughly 2.6 to 3 m (8 to 10 ft) at most. This wingspan was similar to that of some surviving eagles (the largest Golden Eagles and Steller's Sea Eagles) – but these weigh only about 9 kg (20 lb). Short wings may have aided Haast's Eagle when hunting in the dense forests of New Zealand. Haast's Eagle is sometimes portrayed as evolving towards flightlessness, but this is not so; rather, it represents a departure from its ancestors' mode of soaring flight and towards higher wing loading and maneuverability. The strong legs and massive flight muscles would have enabled the birds to take off with a jumping start from the ground, despite their great weight. The tail was almost certainly long (up to 50 cm (20 inches), in female specimens) and very broad, further increasing maneuverability and providing additional lift. Total length was perhaps up to 1.4 m (4.7 ft) in females, with a standing height of around 90 cm (about 3 ft) tall or even slightly more. Haast's Eagle preyed on large, flightless bird species, including the moa which was up to 15 times its weight. It attacked at speeds up to 80 kph (50 mph), often seizing its prey's pelvis with the talons of one foot and killing with a blow to the head or neck with the other. Its size and weight indicate a bodily striking force equivalent to a cinder block landing on the target from a height of 25 m. The eagle had power in its talons easily sufficient to snap a human's neck, or puncture the skull. Its large beak was used to rip into the internal organs and death was induced by blood loss. In the absence of other large predators or scavengers, a Haast's Eagle could have easily monopolised a single large kill over a number of days.
Early human settlers in New Zealand (the Māori arrived about 1,000 years ago) also preyed heavily on large flightless birds including all moa species, eventually hunting them to extinction. This caused the Haast's Eagle to become extinct around 1400 when the last of its food sources were depleted. It may also itself have been hunted by humans: a large, fast bird of prey that specialised in hunting large bipeds may have been perceived as a threat by Māori — for a creature that could kill a moa weighing 180 kg (400 lb), an adult human may have been a viable prey alternative.
A noted explorer, Charles Douglas, claims in his journals that he had an encounter with two raptors of immense size in Landsborough River valley (probably in the 1870s), and shot and ate them. These birds might have been a last remnant of the species, but this is very unlikely; there had not been suitable prey for a population of Haast's Eagle to maintain itself for about half a millennium, and 19th century Māori lore was adamant that the pouakai was a bird not seen in living memory. Still, Douglas' observations on wildlife are generally trustworthy; a more probable explanation, given that the alleged three-metre wingspan of Douglas' birds is unlikely to have been more than a rough estimate, is that the birds were Eyles' Harriers. This was the largest known harrier (the size of a small eagle) — and a generalist predator — and although it is also assumed to have gone extinct in prehistoric times, its dietary habits alone make it a more likely candidate for late survival.
Until recent human colonisation, the only terrestrial mammals found on New Zealand were three species of bat, one of which has recently become extinct. Free from mammalian competition and predatory threat, birds occupied or dominated all major niches in the New Zealand animal ecology. Moa were grazers — functionally similar to deer or cattle elsewhere — and Haast's Eagle hunters, filling the same niche as top-niche mammalian predators such as tigers or brown bears.
DNA analysis has shown that this raptor is most closely related to the much smaller Little Eagle as well as the Booted Eagle (both recently reclassified as belonging to the genus Aquila.) and not, as previously thought, to the large Wedge-tailed Eagle Thus, Harpagornis moorei may be reclassified as Aquila moorei, pending confirmation. H. moorei may have diverged from these smaller eagles as recently as 700,000 to 1.8 million years ago. Its increase in weight by 10 to 15 times over that period is the greatest and fastest evolutionary increase in weight of any known vertebrate. This was made possible in part by the presence of large prey and the absence of competition from other large predators.