The Wairoa District Council is calling on the community to exercise care when disposing of chemicals, following a contamination incident involving a herbicide at the Opoutama wastewater treatment plant.

The Opoutama plant is designed to treat household wastewater – not chemicals. When herbicides, pesticides, paints, solvents, fuels, or dye products are poured down drains or gully traps, they can severely damage the treatment process and harm the coastal environment.

Recently, a bright tracer dye product was detected in the treatment system. On its own, this tracer dye is generally eco-friendly and safe when used correctly. However, in this case, the dye appears to have been mixed with a herbicide, and it is the herbicide component that has killed the good bacteria the plant relies on to clean wastewater.

The contamination has effectively shut down the treatment process, and the cost of remedying the herbicide pollution is money that could have been allocated more effectively elsewhere.

Wairoa District Council Chief Executive Matt Lawson said this is a real setback for the small coastal community.

He explained the Opoutama plant relies on living bacteria to treat wastewater safely. The herbicide contamination has effectively wiped out the biology of the plant.

“This type of contamination is toxic to the good bacteria that operate the treatment system, with the bacteria likely to take two to four weeks to regrow.

“To restore the treatment process, the contaminated contents will be removed, followed by re-seeding and re-establishing the biological treatment. Contractors have been engaged to begin recovery work as soon as possible.

Mr Lawson urges the Opoutama community, and all those connected to reticulated wastewater systems, septic tanks and on-site systems on individual properties, to be extremely careful with chemical disposal. “We remind all residents, visitors, and property owners that no chemicals such as herbicides, pesticides, oils, solvents, fuels, dyes, or paint wash-out should ever be poured into sinks, drains, or gully traps. The wastewater networks and the biological processes in on-site systems are simply not designed to handle these materials, and, as we are now seeing, this contamination has caused severe infrastructure damage, environmental risks, and an unnecessary added cost for the community. It's also an important reminder of how we can all help protect our infrastructure and marine environment through responsible disposal practices and being an environment conscious community. If there is ever any doubt, please contact Council to understand effective ways to dispose of chemicals.”

An investigation is underway to determine how the chemical substance entered the Opoutama system. Anyone with information is encouraged to contact the Council.

7 November 2025

Back to Archive