The wastewater is also tested for MDMA, also known as ecstasy and "the party drug". That amount has increased since early 2019, with the highest of 86g consumed during a week in July 2020.
Cocaine is also tested for, but none at all was found in Whanganui's wastewater in the second half of 2020.
• READ MORE: Kicking meth: Locals seek help to get off drug
Those three drugs were chosen for testing because they caused the most harm, were readily available and were easy to test for, a police spokesperson said.
Methamphetamine was a big concern to police. Current pricing is at $500 per gram.
"Criminals who peddle illicit drugs to members of our community often accumulate significant assets, and in doing so cause incredible harm.
"Methamphetamine destroys the lives of users, as well as harming the friends, family and community around them."
Addiction can also fuel crime - most often burglaries and shoplifting.
Police want to hear about illegal drug use. Phone 105, or talk anonymously to Crimestoppers on 0800 555 111.
People who want help with addictions can contact the Alcohol and Drug Helpline, 0800 787 797, or free text 8681.