In South Africa the Niemann surname is a derivative of the surname of the progenitor in South Africa, namely Johann Niemann aka Jan Niemand.
Due to the fact that many of the earlier family were illiterate, the surnames were usually written by other people, and each spelt the surname the way they thought was correct.
Different forms of spelling the surname that occurred were Nimand, Nimant, Niemant, Neeman, Neemand, Neemand etc.
At one point certain branches of the family started to standardise on a specific spelling, and today in South Africa there are 4 different accepted spelling forms of the surname eg. Niemandt, Niemand, Niemann and Nieman.