Do you know what America did to reduce crime and the homicide rate? We banned lead in paint and gasoline. Yes, seriously!
So, why doesn't Australia do this?
If the homicide rate dropped by 30%, and 30% of that is the result of banning guns, then that means that 0% is attributable to dropping lead levels. This leads to four possibilities:
1) Australia never actually had any lead. Even back in the 1960's, Australia did not use lead paint or leaded gasoline. Possible, I suppose. Maybe gasoline was not used, as adults rode kangaroos to work, and the kids rode wallabies to school? Maybe nobody painted their houses with lead paint, as there is no need to paint corrugated tin?
2) Australia still has not banned lead. If so, they need to do so immediately!
3) They DID ban lead just like the US, but decreasing lead levels did not impact Australian physiology the same way it does in the US. Maybe the mass quantities of alcohol consumed over there somehow rids the body of lead. Maybe being upside-down all the time, and the fact that lead is heavy, meant that all the lead leaked out of the tops of your heads.
4) They DID ban lead just like the US, and most/all of the drop in homicide is due to declining lead levels, and the gun ban has actually had little to no impact on the homicide rate. Coincidentally, this would ALSO neatly explain why the knife and club murder rates also dropped.
So, which theory do you support?