One of my favourite excuses for the “high” NZ dollar is currency reserves being built up overseas (in order to “keep their currency low” they need to buy up foreign dosh, building up currency reserves you see). It is an argument the US likes to make without actually doing any analysis as well.
However, work has been done … a while back.
The linked to paper found that reserves were not excessive anywhere, expect China. A good point to keep in mind no doubt.