Many drug-related offences raise the issue of possession. Possession is defined as having control over the disposition of a substance or thing, and includes having joint possession of a substance or thing [see Controlled Substances Act 1984 (SA) s 4(1) for definition]. A defendant must know that the drug is in his/her custody and control, and that it is a drug to which the Act applies, although he/she need not know precisely what the drug is [see s 33P]. Prosecution must show that a defendant knew that possession of the drug in question was contrary to the laws relating to illegal drugs, though not necessarily contrary to any particular Act.

There is much case law about the nature of possession. There are many cases where a defendant may have some defence on the basis that his/her association with the substance does not amount to possession. For example, where drugs are found in joint living areas of a shared house, the issue of possession may arise. It is also a defence to prove an honest and reasonable mistake about the nature of the substance possessed [see R v GNN (2000) 78 SASR 293; [2000] SASC 447; Criminal Law SA (Lunn) for relevant case law].

The definition of possession in accordance with section 4(1), came into effect on 3 December 2007, and applies to offences which occurred on or after that date [see Controlled Substances (Serious Drug Offences) Amendment Act 2005].