How can i join the U.S Marine corps?
Non-citizens are eligible to enlist in the military but not be commissioned. A non-citizen that is eligible must meet certain requirements: (1) Have an Alien Registration Receipt Card (stamped I-94 or I-551 Green card/INS Form 1-551), (2) Have a bona fide residence established, and (3) Have established a record of the U.S. as their home. Some non-citizens from countries with a reputation toward hostility toward the U.S. may also require a waiver. The federal government cannot petition on behalf of an illegal immigrant so that they can obtain legal status and be able to enlist. The illegal immigrant must go through the immigration process of the USCIS (previously known as the INS). Once the status of the illegal immigrant becomes legal they can go ahead and start the enlisting process. Another requirement is that the Green Card and/or visa must be valid for the entire period of their enlistment.
As a NZ citizen you can not just go and join the U.S marine core after your time in the NZ army is served you can request a transfer or you can attempt to get your US citizenship but that can take a while, or you can join similar specialist units in the Australian military or in England where gaining citizenship will be easier as we are Commonwealth countries. P.S but be aware the U.S military receives much much lower pay and benefits than those of Commonwealth countries.