Wow, a 15 kg bird with a 3 meter wingspan would be quite scary.

Apparently the availability of animals like moa, extinct flightess birds weighing hundreds of kilos with few or no other predators, put a lot of evolutionary pressure to grow big to fill the niche.

Apparently these birds evolved from relatively small eagles who accidentally ended up in New Zealand less than 2 million years ago, then evolved to a larger size faster than almost any known vertebrate, growing to be like 10x the size of their ancestors.

It seems once humans arrived in New Zealand around 700 years ago, the eagles relatively quickly disappeared.

Evidence suggests that moa were not quite as depicted, with their necks fully extended all the time, but more lowered. They may have used that to get to higher branches but not when walking. (Much like emu and cassowary rather than an ostrich). That said moa could be huge. I've stood next to skeletons of a few of them and they were bigger than me.

The usual claim is that Haast's Eagle fed mostly on moa. I have wondered whether it fed also on seals and larger sea birds. The other problem is that most of New Zealand was covered with dense bush and forest, and probably moa lived in that, rather than open grasslands in much of their range (like ostriches), coming out into the open on large river beds and lake shorelines to drink. Most of New Zealand's present-day grasslands have been created by humans (especially since the 19th century.)

That said, I do believe the stories of them attacking humans. If they were apex predators and unused to humans, they may have seen them as potential prey.