A kirpan is a small sword or dagger, worn in a sheath on a strap or belt.
In August, a Sikh man was removed from an Air New Zealand flight from Christchurch to Invercargill, after another passenger noticed his kirpan.
Despite those problems, Peters said changing the Crimes Act to allow Sikhs to carry the kirpan would be a rule to favour them over others.
"If Sikhs can do this as part of their customs, then what do we say to Maori who want to carry their traditional weapon, a taiaha, or perhaps a Hindu who wants to carry a trident, their traditional three-pronged spear.
"Perhaps it's time for a serious talk with the Sikh community to avoid people walking around with a dagger of cold hard steel."