This is a difficult question to answer fully, because it depends on so many different factors, and is across two specialties.

Firstly, population stats according to that holy grail of sources, Wikipeda(if anyone can direct me to better pop stats, please do.) The Māori population in 1800 is est. to have been between 100-120,000 people. The indigenous Hawaiian(hereafter I'm gonna write iHawaiian) population is est. to have been around 260,000 at the same time. Those who claim full blooded iHawaiian ancestry number 140,000. The same among Māori is 280,000. The totals, those who claim both full indigenous heritage, as well as mixed, are 750,000 for Māori, and 400,000 for iHawaiian.

The iHawaiian population took a nose dive after first encounters with Europeans, dropping from that 260k to just under 40,000 by 1900 according to the US census in that year. Likewise, Māori had a huge drop in population, falling as far as 50,000 in the 1850s. Unlike the iHawaiians, this was not only due to disease, but also because there were a series of wars that killed tens of thousands of Māori.

I'm not sure about iHawaiian history here, but at this point in time, the Māori were considered to be 'a dying race' in New Zealand. It was assumed that by the early 20th century, Māori would have died out entirely. After the wars though, there weren't really obstacles in Māori population growth. Social welfare through the late 19th/early 20th centuries meant that Māori received better medical care and education about health.

None of this explains how you go from 50,000 Māori to 750,000 in a century and a half though! And this is where this topic gets really difficult, both for Māori and iHawaiian discussions.

Statistically, all we know about these population numbers was achieved through the Census in both countries. The Census is usually pretty accurate in numbers, but on the finer detail it can be misleading because of self selection bias. What this means is that someone who is Māori/iHawaiian might not name themselves as such on the Census. This could be for any number of reasons: not knowing, not identifying with the group, being ashamed of their heritage, hiding their heritage, or any other. By the same token, someone who has one Māori/iHawaiian great-great-grandparent could identify as that ethnicity because they have formed a connection with that culture in their lives, and it shapes their identity.

For this reason, the total Māori population of 755,000 includes people who would identify as both Māori and Pākehā through intermarriage between the populations. The numbers who claim full blooded iHawaiian/Māori heritage are much lower than the totals, and this could be down to several reasons too: Māori have a higher population because they had access to better medical care, or because more Māori chose to marry within the population than iHawaiians did. It seems to be a sign of the Māori population recovering quicker than the iHawaiian one, and the main thing that stands out to me is that Māori were living alongside a steadily growing European population from 1840 onwards, while Hawaii wasn't annexed until 1898. This may have had some effect. I'd ask this over at /r/AskSocialScience or /r/AskAnthropology tbh, you might get some better answers there!