In Canada and the US, "equivalency" may seem more familiar than "equivalence" because it appears in the name of the certificate that people may earn instead of a high school diploma. The General Equivalency Diploma is often referred to by its initialism, GED.

Both "equivalency" and "equivalence" are nouns so the terms are, so to speak, equivalent (or equal).

I admit that I'm not an expert on New Zealand English, but I don't see any reason to change the author's original wording.