A few things from the firearms act which hopefully answer this for you...
50A Unauthorised manufacture of firearms(1) A person who manufactures a firearm is guilty of an offence under this subsection unless the person is authorised by a licence or permit to manufacture the firearm.
...
(5) In this section:
"manufacture" a firearm includes assemble a firearm from firearm parts.
Follow the link to the description of 'firearm' gets you this:
"firearm" means a gun, or other weapon, that is (or at any time was) capable of propelling a projectile by means of an explosive, and includes a blank fire firearm, or an air gun, but does not include anything declared by the regulations not to be a firearm.
"firearm part" means a barrel, breech, pistol slide, frame, receiver, cylinder, trigger mechanism, operating mechanism or magazine designed as, or reasonably capable of forming, part of a firearm.
Obviously a magazine isn't 'capable of propelling a projectile...' etc so isn't a classified as a firearm itself. The way the reads to me is you could make a magazine for an existing, registered firearm.
That's my interpretation, but a call to the NT firearms registry wouldn't hurt.