However, the judge's earlier statement that the low purity of the drug in the mixture was not to be given significant weight when assessing the applicant's culpability was, in my view, incorrect. Obviously, the legislature has chosen to treat any drug that is part of a mixture as though the whole of that mixture constituted the drug of dependence. That is clear from the fact that it is the weight of the mixture that determines whether the offence is to be characterised as involving, for example, a commercial quantity, on the one hand, or a large commercial quantity on the other.
There is no reason, in principle, why the fact that the mixture contains what is plainly only the most miniscule quantity of a drug of dependence, and is therefore unlikely to produce the deleterious effects normally associated with a much larger amount of that particular drug, albeit in a mixture, should not be regarded as a significant factor to be taken into account in assessing the gravity of the offending.
This very point was considered by the Western Australian Court of Criminal Appeal in R v Mahasay.[26] There it was held that the low level of purity of the methylamphetamine in question was a factor of some importance, at least in a case involving trafficking in that drug. In the case of couriers, the purity might be less relevant.
The matter arose again, before this Court, in R v Minh Thanh Do.[27] There, the Court found it unnecessary to determine whether the low level of purity of a drug as a mitigating factor had survived the rejection by this Court in R v Pidoto & O'Dea[28] of a harm-based system of classification of drug offences.
In my opinion, there is nothing in Pidoto which requires the low level of purity of a particular drug in a case such as the present to be given little or no weight. Whether one views such a matter as a mitigating circumstance, or rather as simply reducing the objective culpability of the offending, matters little in the ultimate result. There is obviously a difference between trafficking in 3.9 kilograms of pure methylamphetamine, and trafficking in 1.9 grams of methylamphetamine in a mixture of 3.9 kilograms. To treat these two offences as relevantly indistinguishable would be an affront to common sense.[29]