Taranaki's regional council admits it made a mistake in not talking to residents near the New Plymouth playground where agrichemicals and dioxin were found.
Stormwater works at Marfell Park unearthed remnants of nine old drums from the former Ivon Watkins Dow chemical plant in nearby Paritutu.
Nine rusted drums and truckloads of soil were removed from the park but no-one told neighbours what was happening.
Four weeks later, Taranaki Regional Council confirmed it had found buried herbicide ingredients, then took four more days to reveal dioxin contamination.
The council says the park is now safe, but admits it would have acted differently in hindsight, given the community concern.
Angry residents have imposed an unofficial closure on the park and say the council's credibility has been destroyed.