Wastewater is currently being tested in Auckland, Christchurch and Whāngarei. Napier and Hastings will be among a mass rollout to 35 sites across New Zealand.
Nash said the programme was not a breach of personal rights, as the results were not linked to a specific person or property.
"This doesn't lead to any home, or anyone like that, so it's not used for enforcement purposes, it's more used for information purposes."
The programme was not to target specific properties, but to get an overall understanding of drug use in the community.
"I'm a firm believer that we've got to treat drug addiction as a health issue as opposed to a criminal issue, but until we actually understand the scale of the problem, it's difficult to know the response we need to put in place."
However, he said the information would also be used to target law and order resources, not just health resources.
"One of the most insidious aspects of organised crime is drugs."
"This will give us the information that allows us to better target law and order resources."
Police Commissioner Mike Bush said the testing was crucial to police understanding of illicit drug use.
"Expanding the programme to regional New Zealand and other major centres will help us continue to build a better picture of the prevalence of illicit drug use in New Zealand communities as well as the subsequent social harm."
The testing will be conducted by the Institute of Environmental Science and Research.
The drugs being tested for are methamphetamine, cocaine, heroin, MDMA, fentanyl and ephedrine/pseudoephedrine.
Testing is expected to be rolled out by the end of the month.
