Grylls stresses that she doesn't want Voices New Zealand to be known as a choir that only does New Zealand music; she takes a special delight in describing the vivid word-painting of Morton Lauridsen's Madrigali and Benjamin Britten's Five Flower Songs, where "you can almost see the slimy roots in the sinewy harmonies of the Marsh Flowers".
Yet the South Pacific is an inescapable presence in Monday's programme, with works by Christopher Marshall, David Griffiths and Douglas Mews' classic Ghosts Fire Water, a "statement on Hiroshima that each generation can take on themselves for whatever they understand their war, battle or struggle to be".
Expect magic when the evening opens with Horo playing his taonga puoro while the choir sing Hildegard of Bingen's O viridissima virga.
"Each music finds its source in a chant," Grylls explains, "but putting them together is tricky. The Hildegard was notated, back in the 12th century, but Horo's music is totally improvised.
We're learning to work together, with a lot of pitch bending and shifting around."
Their collaboration is a process.
"It goes back to 1993 when we sat in the same room as Bub Wehi and her Te Waka Huia singers at Sing Aotearoa and simply sang to each other. What Voices is doing now comes at the end of a 17 or 18-year journey."
Horo has taken his music around the world, playing with both the Weimar Staatskapelle Orchestra and at the 90th anniversary commemorations of the Battle of Passchendaele.
On the lighter side, he has toured with Moana and the Moahunters and this year was part of Charlotte Yates' Auckland Arts Festival production of Ihimaera.
Grylls is musically smitten. "Horomona talks so beautifully about the voices of his instruments," she says. "He's in Helen Fisher's Pounamu as well as David Hamilton's new Karakia of the Stars which has a part specially written for him.
"We have the same respect, passion and sensitivity to the music that we bring together. We don't claim to have the answers yet, but we're on a journey. Having someone like David Hamilton write something for the two worlds, as it were, is both a challenge and a risk and it's got to be done."
Performance
What: Voices New Zealand
Where and when: Auckland Town Hall, Monday at 8pm