Chemicals used to make the illegal drug methamphetamine are causing hundreds of thousands of dollars of damage to sewerage systems, a water quality scientist says.
The scientist discovered the problem when unexplained breakdowns at one North Island treatment plant stopped after police busted a meth ring.
Tara Okan from water monitoring firm DCM Process Control says waste from meth labs can degrade pipes, produce explosive gases in pump stations, kill bacteria in treatment ponds and threaten workers' health.
Mr Okan says he saw one case where the chemicals killed bacteria used to digest waste in a treatment pond, killing all the microbes in the plant and turned clear water into thick, white milky liquid which was then discharged into the sea.
He said the problem only affects smaller towns where the chemicals are not diluted by large amounts of waste water.
Mr Okan said police considered tracking chemicals through sewers to find illegal labs, but scrapped the idea because of worries the criminals might dump hazardous chemicals in rivers or bury them.
He said some plants have put monitoring equipment in place so any chemicals from P labs can be diverted.