PiHKAL actually was banned here. It no longer is, which should indicate that the situation may be more nuanced than I think you are assuming.
In general, if a works fails the “community standards of decency” test, then most people won’t have a major issue with it being restricted.
It’s not hyperbole to say that instructions for knock off gun production would be viewed by most Australians as not far up the scale from child porn. Bomb and chemical weapon instructions are in the same category.
There are problems with the systems of content classification (how censorship is implemented) used in Aus, eg. There was not a 18+ rating for video games until recently, but limiting access to these files is not one of them.
