So I've read that the Māori arrived in New Zealand around the 1300s, while the Australian Aboriginals arrived in Australia around 50 000 years ago. Is there a reason it took so long for New Zealand to be inhabited?
It's a matter of distance. About 60,000 years ago, it was possible to walk most of the way from Eurasia to Australia. Sea levels were much lower, and many of the present-day islands were part of the mainlands. For example, the Sunda Shelf was dry, and Sumatra, Java, and Borneo were part of mainland SE Asia. There were water gaps that needed to be crossed, either relatively small gaps from the mainland and then island-to-island until Timor was reached (the possible southern route for the the first settlement of Australia), or a short hop from the mainland to Sulawesi, and then island-hopping to western New Guinea, which was connected by land to Australia (the possible northern route). The biggest gap along either of these two routes was about 90km, from Timor to Australia. While it would not have been possible to see the Australian mainland from Timor (even with the lower sea levels of the time), there were offshore islands that should have been visible from Timor, and crossing from Timor to Australia was possible without ever being out of sight of land. The northern router, through Sulawesi to New Guinea, has a shorter longest gap, and it was possible to sail along that route without passing out of sight of land. Being able to keep land in sight makes for much safer sailing, since return to the starting point is much simpler than if one goes out of sight of land.
New Zealand is much further away from other land, being about 1000km from the closest land. New Zealand is over 2000km from Australia, Tonga, and New Caledonia. The closest of the Kermadec Islands is a mere 800km away, but the Kermadec Islands themselves are distant from nearby land. This makes exploration difficult! How many days will one sail into the unknown, with no known land ahead?
New Zealand was reached from Polynesia, and thus had to wait for those islands to be settled. This took time - Tonga was settled perhaps about 1000BC, and settlement slowly spread further east, with the Society Island settled about AD1000 (Tonga to the Society Islands is the approximate region from where New Zealand was probably settled). Once eastern Polynesia is settled, the people were, by necessity, accomplished sailors, and it didn't take much longer for New Zealand to be settled.
In a splendid piece of experimental archaeology, two replica canoes sailed from Polynesia to New Zealand in 1985. The sailing of the Hōkūle‘a from Raratonga to new Zealand, as reported in
covered a distance of over 2500km in 16 days. They had the advantage of knowing where they were going - they weren't sailing into the unknown. An exploratory trip over this distance needs to be able to return to the starting point in case no land is found, and finding New Zealand in the first place requires being able to make month-long voyages.
Further reading:
On the settlement of Australia:
On the settlement of New Zealand: