The NZX has minimum requirements which must be met at time of purchase. If price goes down you don't have to top up but company can compulsory acquire. Its a sliding scale but I view it as $500. I have seen this happen. Even took advantage of it once.

When I started, my minimum purchase was $5k. I have now increased to ~$10k. I think with ASB, after $15k there is no economies of scale so if I wanted to enter a new stock and hold more than than, i would probably stage my entry. I haven't needed to this this so far as any holdings above $10k have been multiple purchases over a long period or time (or share price appreciation).