I thought all passports were connected and that somehow that would cause the SA passport to become invalid.
I'd lose my Dutch citizenship should I take up NZ citizenship (unless I marry a Kiwi before then - which I think is discrimination against singles). I definitely wouldn't be able to renew my passport (I think you need to sign for not having taken up a different nationality) and thought it might be considered fraud if I'd try to use my Dutch passport - sure at least at the Dutch border they would see? However, I can't seem to find an explanation for this.