New Zealand is officially a party to the Mercenaries Convention after the government signed the international treaty on Thursday.
Speaking from New York, Foreign Minister Phil Goff says under the convention, it is illegal to recruit, use, finance or train mercenaries in New Zealand.
He defines a mercenary as a person paid to fight and kill for money.
Goff says there is not widespread evidence of New Zealanders being signed up as mercenaries, but he can't rule it out.
He says the convention does not prohibit people being hired for policing or security work overseas, from joining a foreign army under standard conditions of service, nor people joining the fight for a cause they believe in.