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airnzuk asked me to send them a DM with my Airpoints number. I did that and this was their reply:
Hi Andrew, if you are paying with Airpoints Dollars, your offer will be made in your local currency and then be converted to New Zealand Airpoints Dollars at the relevant daily exchange rate for New Zealand Dollars. ^LM
Not exactly helpful. I asked them for a link to the daily exchange rate they use.