Those of us who grew up in a harmony singing culture can pretty much do harmony "spontaneously", or at least fake it. There's lots of hymns I know from my youth that sound just as impressive as this waiata when sung by a large crowd of people.¹

I once sang 606 in a crowd of over 500 congregants, in harmony, and the experience was ... well, let's just say shattering.

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¹ music starts at about 1:00.