Agricultural poisons have been found in rusting drums buried in a park in New Plymouth.
Stormwater works a month ago uncovered one drum and the corroded remains of another in Marfell Park.
Up to seven more containers were found this week in the former landfill.
Taranaki Regional Council says tests identified the herbicide ingredients tetrachlorobenzene and trichlorophenol.
Environment director Gary Bedford says the toxins had not previously been detected in groundwater despite 15 years of testing.
Scientists previously tested once for agrichemicals, nine years ago, after suggestions that waste from the Ivon Watkins Dow chemical plant was dumped there.
Mr Bedford says more detailed environmental testing is now focused on several specific chemicals.