There's a little distance between my knowledge base today and that I possessed when I majored in
physics in
college, but I'll try to
dredge up the important bits.
It is possible to extract u235 from u238 ore by using a complex gaseous-diffusion process involving a compound of uranium and fluorine. I don't know if this is what factgirl meant when she said no "ordinary" process. The two isotopes are chemically identical, but their different masses allow separation to take place. u238 is fissile, but you need some really excited neutrons to keep a chain reaction going, so it's just not practical.
The minimum size a u235 chunk must be in order to support a chain reaction is a sphere of 18 cm weighing 53 kg (or about 117 pounds). That's a lot. (You can get smaller devices from plutonium, which does not occur naturally.) You'd need to refine more than 16,600 pounds of ore in order to hope to get enough u235 for one bomb. And even then, unless you're pretty smart and can successfully nest shells of u235 and u238, you'll get a fission bomb, which will be nowhere near as powerful as a thermonuclear fusion bomb--something on the order of one-thousandth the potential power of a fusion bomb.
My conclusions: If the cult did go down this path, they weren't very smart. Any device they could have potentially made would have likely been big and bulky, and Western Australia is probably about the only place they'd be able to detonate it with any secrecy. But I think it's extremely unlikely that they built a device on-site, as the industrial requirements are pretty major. Plus, I gotta believe it's easier to buy a bomb from the former Soviet Union than to build one. I read a report in 1991 that suggested that a radical mafia faction known as The Star bought one, though I don't know why.
And that's conclusion #2--I think that any organization that 1) had the wherewithal to buy a bomb and 2) the desire to use it, would have already done so. Consider Hamas, which basically has the full financial support of the entire Syrian government. If it was easy to buy and move a bomb, wouldn't they have done it already, at a time when the Soviet Union's borders were falling apart and no one was looking? If I was out to buy a bomb, that sure as hell is when I would have done it.