No, Gumbercules. I don't now about Australia, but the vast majority of US criminal laws do not apply extraterritorially. It's not just that the US doesn't bother prosecuting people who use drugs outside of the US; they couldn't even if they wanted to. US law applies in US territory, and not elsewhere, except in certain limited cases.
One exception I'm aware of is that US citizens or permanent residents who have sex with child prostitutes in other countries can be tried in the US, and most likely treason or similar crimes would apply to US citizens overseas, but that's me speculating.
However, almost all other US laws apply only in the US. Extraterritorial laws are the rare exception and not the rule. If you smoke pot in Amsterdam, that's between you and Amsterdam. If you are 18 and get drunk in Puerto Rico (where it is legal), your home state won't go after you.
And, taken to the extreme, even serious crimes like murder are typically only crimes under the jurisdiction in which the murder occurs. If you go kill someone in Mexico, it's Mexico whose law you have violated and Mexico who will punish you. Now, if you go to the US after murdering someone in Mexico, the US will arrest you and send you to Mexico to face justice under Mexico's laws -- but only if Mexico asks. Extradition is not the same as extraterritorial application of law.
Here's a document with information on US extraterritorial application of law, considering it from constitutional, statutory, international law, and lots of other perspectives. Piracy (real piracy; guys on ships with guns or other "stateless vessels") is an extraterritorial application of law that had slipped my mind in my first comment. A few other cases seem to be things like, if you kill the President (or another high-ranking federal official) when he's in Japan, the US will have some beef with you even if Japan doesn't care. But these tend to be limited. Also, it's almost all federal criminal laws that apply extraterritorially; state criminal laws (which are the vast majority of criminal laws in the US) almost never do.