Should have left the country and moved to Germany while he had the chance. Afaik Germany doesn't extradite its own citizens. Question to the legal experts here

Should have left the country and moved to Germany while he had the chance. Afaik Germany doesn't extradite its own citizens. Question to the legal experts here: Assuming he manages to board a German ship and to enter international waters (while officially not allowed to leave NZ). Would this be sufficient, or would international treaties allow NZ or the US to intercept that ship?