If illicit drugs are found among your belongings, and if you didn't know they were there, the onus is on you to prove that you didn't know. This is the norm throughout the English-speaking world and, in particular, is stipulated by s.5 of the Drugs, Poisons and Controlled Substances Act in the state of Victoria, Australia. The same Act defines possession of a drug "for sale" as trafficking, without quibbling about whether the possessor is the seller [s.70], and defines possession of a traffickable quantity as prima facie evidence of trafficking [s.73(2)].
In 2008, Vera Momcilovic was convicted of trafficking under that Act, notwithstanding the uncontested admission by her live-in boyfriend that the drugs were his and that he was the trafficker.
Momcilovic appealed to the Supreme Court of Victoria (sitting as the Court of Appeal) on the ground that the reversal of the onus of proof was inconsistent with the presumption of innocence as recognized in s.25 of Victoria's Charter of Human Rights and Responsibilities ("the Charter").
In its judgment, delivered on March 17, 2010, the Court remarked that the reversal of the onus of proof was "not so much an infringement of the presumption of innocence as a wholesale subversion of it." Nevertheless, the Charter did not enable the Court to reverse the conviction, although the Court acknowledged an error in sentencing and accordingly suspended the rest of the sentence. On the conviction itself, the Court could only issue a "declaration of inconsistent interpretation", indicating that the Drugs, Poisons and Controlled Substances Act could not be interpreted in a manner consistent with the presumption of innocence, and that there was no demonstrable or reasonable justification for the breach of that presumption. The legislators are not obliged to amend the offending law. [The Australian Human Rights Law Resource Centre has a useful summary of the Court's judgment, with a link to the full text.]
These facts were fairly reported on the website of the Melbourne Herald Sun on March 18. But in the printed edition next morning, under the headline "Drug dealer out of jail", the same paper lived up to its "Herald Scum" moniker by omitting all references to the boyfriend and giving the last word to "Lawyers for Attorney-General Rob Hulls", who argued that (in the paper's words) "onus on an accused to prove they did not possess the drugs was essential to the successful prosecution of trafficking."
Excrement. The reversal of the onus of proof is a de-facto immunity for any trafficker who can stash the merchandise among someone else's belongings, and a standing invitation for traffickers and corrupt officials to discredit potential witnesses by planting evidence on them. In combination with the notorious lack of security at Australian airports, it was a standing invitation for traffickers to infiltrate the ranks of baggage handlers and customs officers, and ply their trade at someone else's risk. (Oh, yes, security was eventually tightened somewhat, but only after a report detailing the problems was leaked to the media; and the author of the report was prosecuted and dubiously convicted for allegedly leaking it.)
In fact the legislative reversals of the onus of proof in drug-possession cases are the biggest drug crimes ever committed. But because the perpetrators were legislators acting ostensibly in the course of their legislative duties, they are immune to prosecution. That's the downside of the separation of powers.
If you want to know what an honest anti-drugs policy would look like, consider three simple realities:
(1) To discourage use of drugs, we need retail (street) prices to be as high as possible.
(2) To discourage trafficking in drugs, we need prices at all other points in the supply chain to be as low as possible, so that concealable quantities are not valuable enough to be worth trafficking.
(3) A bottleneck in the supply chain (e.g. due to law enforcement) reduces prices upstream and raises prices downstream.
When the problem is thus stated, the solution is a no-brainer: put the bottleneck at the retail level -- that is, concentrate law-enforcement efforts on the street sellers. Interdiction at any other point in the supply chain is counterproductive in that it sends the wrong price signals. Upstream of the retailers, enforcement should be just strong enough to maintain the need for concealment: the risk of confiscation, or of prosecution if caught in the act of selling, is enough; the risk of prosecution for mere possession is too much. Meanwhile the retail customers (drug users) must not be at risk of being prosecuted for possession or purchase, because that discourages them from informing against the retailers!
Together, these points imply that possession of any quantity of drugs, at any point in the supply chain, should not involve a risk of prosecution. Does this mean possession should be legalized? No. My recommendation is that possession or purchase of any quantity should be a summary offense punishable solely by confiscation of the contraband, with no conviction recorded, so that prosecution would be possible in theory but pointless in practice. This would neatly solve the problem of unwitting possession. If the drugs are yours, confiscation involves a loss. If they're not, it doesn't. Either way, justice is done.
The same reform would greatly increase the deterrent for selling drugs, especially at the retail level, because every buyer could inform on the seller without admitting any more than possession and purchase. At the retail level, every junkie would be a protected informant, and the sellers wouldn't stand a chance. If that were not enough, the legislators could create an offense of being a "habitual" supplier of drugs, so that a seller could be convicted on the evidence of a large number of buyers each of whom was the sole witness of a sale, without the need for two witnesses to any particular sale.
This optimal anti-drug strategy does not involve any need to prosecute people for possession at all, let alone to reverse the onus of proof.
But of course the legislators will want nothing of this, because they're not in the business of solving problems. They're in the business of exploiting problems to win elections, so that they can implement the policies demanded by our real masters, namely (a) big political donors, (b) other vested interests with large PR budgets that can influence election outcomes, and (c) religious demagogs who tell their followers how to vote.