Note: I'm into heraldry and not vexillologie. And I exaggerate. :-)
I think flags shouldn't have detailed figures like kiwis or stars, but should instead consist of geometric figures like bars, lines, or spirals. Here is why: Imagine a waving flag as on the image. But you usually see flags from far away, so the details get blurry for the viewer. Plus, you rarely see one straight from the front, so the waving flag partially hinders the view on itself. It may look like this to you (bad quality and over the top photoshopping intended):
The stars of the hemisphere will get unrecognizable from a less then ideal perspective, if you do not know them and will just appear to be some stars … or circles, dots? Also, can you tell whether this is a spiral or a kiwi on the right flag? Stick to the easy design.
I like the top right flag and I cannot think of any country's flag that is just black and white, so I would take that. The bottom left fern design would also be nice if the number of leaves is not fixed (so smaller variants don't look bad because of too many tiny things - I'm looking at you USA!)