No personal experience, but expect the same difficulties as that present themselves when purchasing the phone - if not more. For warranty issues, you’ll need to use a proxy in Europe - from the terms and conditions of sale:
5.2 The Products that we currently offer will only be delivered within the European continent and warranty related assistance can only be granted if requested from one of the following European continental countries: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Holy See (Vatican City State), Hungary, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, The Netherlands, Norway, Poland, Portugal, Romania, San Marino, Slovakia, Slovenia, Spain, Sweden, Switzerland, UK.
In effect, you’ll need to arrange (and pay for) shipping of parts and/or returns between Europe and NZ (and vice versa) yourself. For warranty claims you’ll need someone in Europe to forward your device (though shipping between them and Fairphone should be covered).