Professor Alex Stevens:

My colleague, Dr. Caitlin Hughes from Flinders University, has done quite a lot of research on this and has shown how fiendishly difficult it is to come up with a simple number that distinguishes use and possession from sale and supply of these substances. The most sensible thing to do, unusually, is to follow the English example. I am not a great fan of British drug policy but it does impose the burden on the prosecution to prove that a person has intent to supply. It is not hard to do that. If a person has scales, a list of customers and lots of text messages on his or her phone that are obviously drug orders, it is pretty easy to prove that he or she is not just in possession but has intent to supply. That is a much more reliable way of distinguishing supply from possession than just setting an arbitrary weight limit. My recommendation would be to impose that burden of proof on the prosecution and not to use an arbitrary weight limit.