I actually had a few purchases showing in my statement on the same date for the same card, just in USD, rather than NZD. They resulted in foreign exchange charges of about 0.87%(as calculated by xe credit card charge calculator) for about half the charges showing on that day. A sample was USD$112.46, which resulted in AUD$148.38. While not particularly great, vits a lot better than my other credit card which has transactions showing up on the same date(Citibank Signature Visa), which had a surcharge % of about 4.5%, but I expected that as that particular card issuer is supposed to charge 3.3% on top of the VISA exchange rate.
As another data point, I also had another purchase on Bankwest Platinum showing on my statement on 2nd April for $USD206.29, which resulted in AUD$271.86, which is -0.322%
If there's significant currency movement, then I would expect to see swings both ways, positive and negative for the consumers, so maybe OP had some bad luck on that day.
There are other variables in play as well here which I don't think any of us really know the intricacies of. I have about 12 transactions in USD which all show up on my statement as 7th April. The breakdown of the surcharge(on top of the xe rate) for them is:
6 purchases made on 31/03 - 0.87%
1 purchase made on 31/03 - 0.85%
1 purchase made on 02/04 - 0.87%
3 purchases made on 03/04 - 0.82%
1 purchase made on 04/04 - 0.82%
So the differences between the surcharges which show up on the same day on the statement could be due to rounding differences, or other factors eg. time of day the transactions were actually settled, etc..
For the 3 weeks I was away in the US and used the Bankwest Platinum card, I did a quick analysis of all my transactions on that card and the surcharge(over XE) worked out to be between -0.5% to just under 1%, but it never went over 1%.
I also had some transactions on the Citibank Signature Visa card, and surcharge worked out to between 3% to 5%.