Publius: Remember too that customs use their own published USD->NZD conversion rate, not the rate you paid, to determine the NZD value of goods/shipping.
Just adding to that -
The rates set by Customs used to be (relatively recently) for imports only updated every fortnight and on top of that the rates were set a week or so prior to that for publishing. I think it is still the same, in which case they can be disadvantageous or advantageous according to how much actual rates have moved in the meantime.
Moving on to earlier posts of others -
GST on the conversion fee has been mentioned. The OP DID get that fee applied zero rated WHEN the currency was bought i.e. it showed on the credit card statement without GST. Just because an input is zero rated that does not mean that it remains as zero rated for outputs (or subsequent transactions in the case of the non GST registered) in the proportion it comprises those nor that it flows into subsequent transactions. The GST system would become impossibly cumbersome and fail if that were not to be so. The fee was part of the conversion cost (in the same way as the banks/credit card spread - the difference between buy and sell - was).
So Customs handled the GST on the imported goods Transaction Value correctly. The only difference is that if events had transpired that they could have used their own table conversion rates, rather than sighting the OP's actual cost from the credit card statement, then that would have not included the conversion fee. However, as mentioned above this may or may not have been a good outcome for the OP.