SG Commonwealth & British Stamps 1840-1979 (2020 edition) notes for the 1963 Christmas stamp SG 817c. Nosebag flaw. The note under an enlarged picture of the donkey's nose states: An orange flaw over the Donkey's nose appears as a nosebag (Pl. 1B, R. 3/8).
Rozie, you may wish to peruse here, if you are sorting your NZ. Be aware, depending on the print process of a stamp in question, the variety may simply be an EFO (Error Freak or Oddity) Printers ink viscosity , registration etc.
I am hoping to add one to this page and see if anyone has any thoughts if this is anything of interest. SC 843 series and SG 1384/8 series. Left stamp appears to have a green top background instead of brown: