As your an Aussie, I'd exclude Kratom, as Australia is one of the few countries in which it is illegal. Would be hard to get on the black market as opiate dealers would concentrate on selling the big guys such as Heroin and Oxy.
Half yearly abstinence is probably helpful, particularly if you get fond of a certain substance. For most of the people here who are addicted it usually starts out as once a month or week, then the addictive mind set says "why not a session mid week, that won't hurt" You can get off the bandwagon now, but chances are you won't want too. Useage escalates to every other day then daily. Your dependent now! You'll want to get off the bandwagon now but can't!
Yeah, I have seen the weekend warrior thing in my friends with certain substances too..
That's exactly why I want to keep it as a bi-yearly thing, at most, because I don't need an addiction this early in the peace lol.