The knot does not appear in the list of prohibited weapons: https://www.legislation.gov.au/Details/C2018Q00081
I don't know about Australia, but in Germany this knot is illegal to own and carry this knot if it is additionally weighted with, for example, metal balls.
Sources:
Wikipedia: https://de.wikipedia.org/w/index.php?title=Affenfaust&oldid=...
Primary source: https://www.bka.de/SharedDocs/Downloads/DE/UnsereAufgaben/Au...
I did find something interesting about "the Slugshot" being illegal in some US states, but I would take this with a grain of salt considering the source.
http://www.oldsaltblog.com/2018/07/the-monkey-fists-nasty-co...
A "self-defense" keychain with a 1 inch steel ball enclosed within a monkey fist knot at the end. At 0:18 the demonstrator smashes a laptop with it. I found that pretty funny.
Is it classified as a weapon or something?
A colleague made one one so we could throw a rope over a beam. We all laughed about dobbing him in, then chopped it off and chucked it into the bin.