Jacinda Ardern made the promise in the wake of Friday's attack at two mosques in Christchurch.
A court in Munich, Germany, said it must be assumed the hunter "will handle firearms and ammunition carelessly in future as well.”
The legislation is meant to deter a family member from returning a firearm to a relative whose gun license has been revoked.
Illinois took away the suspect's guns after he was deemed a risk. His father gave them back.