One likely impact on New Zealand use is the availability of other drugs. The much more alarming rise in methamphetamine appears to have, anecdotally at least, resulted in a decline in marijuana in some communities.
Massey University Illicit Drug Monitoring System (IDMS) showed the change occurred between 2015 and 2016 as meth became cheaper and easier to access. The report found a sharp decline in the availability of cannabis during the same period.
To be blunt, what hasn't traditionally impacted rates of use is law and enforcement. One WHO study in 2008 included data from New Zealand as representing Oceania and found: "Globally, drug use is not distributed evenly and is not simply related to drug policy, since countries with stringent user-level illegal drug policies did not have lower levels of use than countries with liberal ones."
Legalisation or decriminalisation in countries such as the US, Canada and Portugal has apparently led to some small increases, although it is hard to tell whether these rises may have occurred anyway. In Colorado, youth use has been stable since legalisation in 2015, interrupting a rising trend.
Justice Minister Andrew Little recently commented a referendum "informed by a thorough factual public debate will lead to a result likely to be more credible and accepted".
Could it be that the spectre of increased use is one of the first fallacies to be weeded out?