If you want to work as a physiotherapist in NZ, you have to be registered - see the official website.
http://www.physioboard.org.nz/index.php?Registration
If you will be in NZ on a WHV, notice that you are not allowed to accept a permanent position without having applied for and obtained a work visa. There are more details about the formalities here.
http://www.immigration.govt.nz/migra...belgiumwhs.htm I haven't heard of a Belgian going through this process before, so can't help you with the time-lag - you could ask INZ themselves.