Hard News: What does the wastewater testing for drug use actually show?

The results from the first three months of national wastewater testing for for drug use were published yesterday by Police and they came with a pretty obvious headline: we're still consuming a lot of methamphetamine in New Zealand – probably even more than we were a couple of years ago.
The estimated 16kg of meth used each week dwarfs the 4kg of MDMA consumed nationwide. The "average proportion of drug use" – the release doesn't explain exactly what this measure is or how it was arrived at, but it relates to the frequency of detection in samples – was highest in Northland, with…