It says, "a firearm that is a rifle or shotgun", and then it goes on to say, “that is capable of discharging centre-fire ammunition in a semi-automatic manner”. Then the third requirement is that the firearm “is designed to accept a detachable...magazine with a capacity greater than five cartridges”. The final clause is, “of the type for which the firearm was originally designed”.
That means that the firearm and the magazine cartridge were designed together by the manufacturer. Compatibility with a third party manufacturer's magazine beyond the original design intent of that firearm would not render that firearm prohibited. That firearm would not be prohibited according to this definition.