Historically, there have been issues with the availability of Australian and New Zealand maps for at least the older models of the built-in nav systems in the Ducato, complained about by a number of Australian motorhomers, so I'm not entirely convinced of the following, BUT the situation might well have changed recently:
The generic TomTom shop shows the availability of NZ mapping, and entering "TP" on this page to check if it is compatible with the TP series indicates it is.
https://www.tomtom.com/en_gb/sat-nav/maps-services/shop/travel-map/new-zealand/
Given that, I think I would at least attempt the purchase/update process set out here:
https://help.tomtom.com/hc/en-gb/articles/360013901640
(Though the route to this info was via a "Map Update" link, TomTom Home has a "Purchase Maps" option, and whilst this will allow you to buy a newer version of the existing (UK/EU) map, if it is supported it should also allow you to buy/install the NZ map. (On a different manufacturers built-in TomTom device, I have gone through the process of buying/installing an additional map)).