High levels of methamphetamine have been found in Whangarei's waste water.
Police say monthly testing for illicit drugs began in August - but until now, the full extent of the town's usage has not been known.
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Now, enough data has been collected to show "a high incidence of methamphetamine in the Whangarei wastewater", Superintendent Russell Le Prou says.
Wastewater Testing and the Rataora Trial, two programmes within Te Ara Oranga, are behind the testing at Whangarei's wastewater treatment plant.
"The collaborative model has been developed in response to increased level of harm from methamphetamine use in Northland communities, to whÄnau and children," Superintendent Le Prou says.
Wastewater has also been tested in Auckland and Christchurch since December 2016. Methamphetamine, ecstasy (MDMA) and cocaine were detected in all cities.
Police hope that by comparing the wastewater methamphetamine levels against how much they're taking from the community, they can understand whether seizure operations are effective in stopping the drug.
"If the wastewater shows less usage in the community then it may be one indicator that the targeting of that operation was successful," Superintendent Le Prou says.
"If no impact is seen it may indicate that further investigation is needed into methamphetamine supply."
Police say testing water is an accurate, cost-effective, timely and non-intrusive method. The police methamphetamine team is also cold-calling suspected users, and performing operations on suppliers.
Heroin was not detected in any of the wastewater samples.