Just had a look online. Firearms, firearm parts and ammo are a no no, doesn't say anything about ammo parts such as primers so I can't see an issue.
From NZ Post..
Air guns
Ammunition
Bullets
Firearms
Firearms partsą
Firearms accessories*
Grenades (inc. imitation ones)
Weapons, including bows, crossbows and arrows, harpoons, spear guns, sling shots, catapults, swordsticks
Stun guns, tasers and stun baton
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