It actually does not define differences between loaded magazine FITTED TO or SEPARATE to the Firearm.
Loaded firearms
(1)
A driver must not operate a motor vehicle on or in which is carried any firearm, airgun, or restricted weapon (as defined in section 2 of the Arms Act 1983) that is loaded with a bullet, cartridge, missile, or projectile, whether in its breech, barrel, chamber, or magazine.
The act was probably designed prior to detachable magazines becoming common place in NZ
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