There were no humans at all in New Zealand until about 1250AD

They did have boats (dugouts), but they weren’t suited for the ocean.

Indigenous Australians were not mariners, they reached australia by island hopping, and at a time when the sea levels were even lowers and thus the distances between islands were lower (Tasmania was a peninsula at the time).

While NZ is close to Australia by air, the Tasman sea is 1700km of open unforgiving ocean. This is not a distance you can reasonably cross as a people with little history of navigation, or even a lot of it. Scandinavia to Iceland is 2/3rds that distance and Iceland was only colonised in the 9th century, by a reasonably maritime culture (mid-era norse).

Indigenous Australians never reached New Caledonia either even though it’s much closer to Australia (~1200km), as far as I know the earliest arrivals were circa 1500~1000BCE at the earliest and from the other side (the Lapita reached New Caledonia from through).

NZ, Easter Island, and Hawaii are most recent.

That's only part of the picture, though. Some bad things happened, but as a consequence, some arguably much worse outcomes were avoided.

The sole reason that slavery and cannibalism do not exist in New Zealand today is precisely because it was colonized. The British outlawed those things (and brought Christianity, which also had an impact).

The "displacement" wasn't extensive, either. The link says the "population may have reached 100,000" prior to colonization, in a nation of 100,000 square miles. Much of it, particularly in the south, was never occupied or controlled, the terrain was neither hospitable nor accessible until colonization brought roads. Now that area is highly productive and exports from there pay for hospitals and schools.

It's also basically unfathomable that if the British hadn't colonized NZ, it would have been totally left to its own devices to develop. Another, less friendly colonizer may have come. What if it were, say, Belgium?

It gets a little tiresome to hear one side of the argument for decades on end, while people fear ostracism for even thinking of mentioning the other half of history.

I agree with you on this, at least. The length of time doesn't matter.

I would argue that the only sensible reason for a feeling of guilt is actus reus. I didn't participate in colonisation, I don't feel guilty that my ancestors did, and I'm yet to hear a clear argument for why I should.

It does seem logical to feel guilty if you benefit from the displacement of existing societies to their disadvantage, are aware of that benefit, and don't work to correct it.

I'd also suspect that the violence and inhumanity inflicted during colonisation are, as far as the "how guilty should descendents of colonisers feel" calculation goes, irrelevant. The thing that many of us are personally responsible for and should feel guilty about is shutting our eyes, and not doing enough to correct the inequity that still exists today.

While the way 1700s British acted would not meet modern British standards, it's a hell of a lot closer to them than Maori behavior and values of the same time period. In the mid-1800s some Maori invaded the Chatham Islands and enslaved, killed, raped, and ate the population (called the Moriori). Here's a quote from Wikipedia:

A Moriori survivor recalled : "[The Māori] commenced to kill us like sheep.... [We] were terrified, fled to the bush, concealed ourselves in holes underground, and in any place to escape our enemies. It was of no avail; we were discovered and killed – men, women and children indiscriminately." A Māori conqueror explained, "We took possession... in accordance with our customs and we caught all the people. Not one escaped....."[38] The invaders ritually killed some 10% of the population, a ritual that included staking out women and children on the beach and leaving them to die in great pain over several days.[34]

If you're interested in learning more you could read some ethnographies of modern-day (or close to modern day) primitive peoples. The Yanomamö work thinks a lot about violence because people (the antrhopology community) couldn't even believe how violent they are. The other way to go about it is to think about the development of morality in humans (like moral universals). One theory goes small gods who are fallible and many --> big gods who always watch and are few --> the kind of normative morality software you have installed right now, which doesn't depend on gods.

But also, while I agree that you shouldn't feel guilty or responsible for the actions of your ancestors, if those ancestors put in place a system of inequality, and some of that inequality persists to this day, which you benefit from while the descendants of those oppressed are still suffering from, don't you think that's at least worth acknowledging? And maybe considering that those who are still suffering because of an unresolved systemic marginalisation don't deserve to?