PAR is only based on the name of the qualification, so it's a rough guess. It's only really of use to people who have no idea whatever if they are likely to be entitled to points or a visa. It can be quite far wrong in some cases, if the foreign college's name for the qualification sounds like a New Zealand course, but the content is very different.
IQA is done by an in-depth look at the subject matter of the course, the time spent and the depth to which it was studied, and the applicant's results. Then, based on all that, NZQA tell a Level, and (if any) the NZ qualification to which the foreign one is most nearly equivalent.