The low notes are not instrumental. They're a Polynesian technique of punctuating a tune with percussive almost grunt-like drops to a lower octave, either by the lead singer or the others. It isn't always done and I can't find an example on YouTube, but I've heard it done the way folksong.org.nz's score has it.

Apart from that, it's a pretty normal Western hymn tune with nothing very traditionally Maori about it.