Well so much for just trying to get the casing shipped over. I got a response from the Weapons Licensing people which says
Information obtained at this office indicates that this item may be contructed from the upper and lower receiver of an M4/M16 carbine / rifle.
This would make the item a category R weapon as provided for in section 8 subsection(1)(a) of the Weapons Categories Regulation 1997.
Should the item not have the upper and lower receivers of the M4 / M16 in built, the item would be considered a replica of such weapon and would also be classified as a category R weapon.
To lawfully possess category R weapons such weapon must be rendered permanently inoperable by a licensed Queensland armourer who will issue a Form 31 "Certicate of Firearm/Weapon Being Permanently Inoperable".
This will also apply to Airsoft weapons irrespective of whether the internal mechanism has been removed or not.
In the instances outlined above, to lawfully acquire these items, the usual Permit to Acquire (PTA) requirments will apply.
You are advised that from previous decisions regarding anything "Airsoft", the Authorised Officer Queensland Police Service Weapons Licensing will not approve the importation of these items.