The legislation doesn't include americium, and even if it did‚ I presume it will be imported under license.

https://www.legislation.gov.au/C2004A03417/latest/text says "Nuclear material means any source or any special fissionable material as defined in Article XX of the Statute." and Article XX only mentions uranium, plutonium, and thorium.

In any case, high-schooler David Hahn showed us what's possible with a bunch of smoke detectors, camping lantern mantels, and the like. https://en.wikipedia.org/wiki/David_Hahn His lab became a Superfund site.