Given the fact that the passenger surrenders control of the baggage from the point of check in, retrieving the baggage at the point of disembarkation it is a sad indictment of legal process that one of the most important legal principles involving a nexus between the accused and the evidence must exist. As the passenger does not control their baggage it is difficult to conclude legally that the accused is in fact guilty at the point of disembarkation.
That is however assuming the law continues to abide by those rules. As Keg alludes to, there have been a number of investigations into carriage of narcotics via domestic flights. I know of such investigations too.
When you consider that all bags can be opened, and often are, no watter what your locking device, is it plausible it was not put there by her? The answer is affirmaitve.
Consider if you will the situation in the United States. A bag can be opened and searched by the "TSA" and the search need not be in your presence. It remains however grounds for legal conviction if something is found, depsite the search being conducted away from the accused. I as flight crew, cannot believe such rubbish is not challenged, it doesn't follow established legal principle.
In assessing the particulars of this case, just remember she lost control at the point of check in...The bag passes through a number of "secure" (my emphasis) areas, through a number of hands to arrive overseas a number of hours later to be retrieved by the accused.
I'm not sure the failure belonged to the airport, QF or anyone else. However, the system could well have failed! Anybody who understands legal principle realises that the necessity to prove the connection between check in baggage and baggage retrieval assumes you are in control of the bag, which you are not. This raises the prospect that reasonable doubt exists........