Centimes On The New Zealand Cover?

Anyone have idea where is added this handstamp, by whom... and why in centimes?

It is a postage due marking. The UPU used a fake currency of gold centimes, so the deficiency was calculated (and usually doubled), converted to the equivalent in gold centimes, and marked on the cover. The recipient would typically have covered the shortfall. The destination country can convert the centime amount into a local currency.

edit to add: I didn't make clear that I believe that the postage due handstamp was added in New Zealand. Based on the position of the existing stamps, I almost wonder if another stamp was originally on the cover and fell off before being processed? A 40-centime taxing is not insignificant.

Thank you for the explanation, CJD. I have seen before such kind of HD on some US covers, but there has been written "C." instead centimes, so I just thought that it was value in US cents.
Auckland postmark looks to be made twice, with a very used and worn stamper, and Due handstamp with a neat one. Maybe I am wrong, but I suppose that 2 stampers probably haven't been carried in the same PO.