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Assuming that NZ has a similar set-up to England, then I would say that these graves are "common" graves, i.e. no individual person or family purchased the grave site. I think that in such instances the cemetery owned the grave -
"Common Graves:
A common grave was a plot which belonged to the owners of the cemetery rather than to a private individual. These plots were used to bury the bodies of unrelated individuals who died over the course of a few days and did not have the means to pay for a plot with private burial rights. These graves were not marked with any kind of headstone and so the occupants were not formally
commemorated though the burial itself would have been paid for by the family of the deceased."
