During his travels in the Amazon in the 16th century, explorer Francisco de Orellana reported about a huge civilization stretching across the Amazon, whose populations dwarfed all of the largest cities in Europe, which he estimated to be in the millions. For hundreds of years, they claimed this has to be an exaggeration because there were no traces of these cities, the "limited technology" of the natives, but more so because soil in the rainforest is actually extremely poor, and to support such a massive population, they would have needed an extensive agriculture system to feed the masses.
In the past 20-30 years, they have actually discovered ruins and networks of these great cities all throughout the Amazon, many of which had been reclaimed and overgrown by the forest. They also discovered that the natives had actually transformed the soil of these areas (likely over hundreds, if not thousands of years), which would have allowed them to implement large scale agriculture greater than anything seen in Europe until the implementation of modern large scale agriculture that we see today, therefore supporting Orellana's claim of the existence of a massive population. Furthermore, the soil they created, known as Terra Preta today, has been demonstrated the ability to improve productivity up to 1000% and bind excess nutrients in the soil for long periods of time, allowing them to potentially and consistently grow crops throughout the year without needing to relocate.
Given the proper management of resources, as well as the fact that Native Hawaiians focused on implementing crops that worked within the Ahupua'a system, I'm not sure why people can't believe they couldn't support a population of up to 800,000 people. As I said: might have really sucked with the amount of resources they actually had access to on the daily, but it is still more likely to be true, given the success of other indigenous civilizations throughout the world that were also able to do so.