If you are planning on buying a non-cellular connected version of the Apple Watch, you should be okay as they should be the same anywhere in the world(aside from packaging language on the box). For service down the line from Apple, as long as you keep your proof of purchase and/or register it, Apple should honor it. I have taken as U.S. iPad to an Apple store in Japan when I ran into issues there and they are happy to help.
If you are buying a cellular connected version then a few things to keep in mind. Make sure to buy the model that is compatible for Germany as well. It looks like New Zealand and Germany have the same model sold so you should be okay there. (Apple Watch Series 3, Model A1889 (38mm), Model A1891 (42mm)) are the ones you want if you want the watch to work with both German and New Zealand carriers.
For reference you can view this page, Apple lists which model is compatible in which Country and their respective carriers in detail.
Also as a side note, Apple watches do not get locked to a carrier, but they are dependent on the iPhone they are connected to (can't just get a cell plan just for an Apple Watch, must be a +1 to your normal cellphone plan since it shared your same number) so you will need to defer to any carrier locks to the iPhone they are paired to.