Half a dozen mature pohutukawa trees in Maclean Park, Paraparaumu Beach, have died because of poisoning, and will need to be removed.
Council parks and recreation manager Alison Law said staff investigated after finding a mysterious substance at the base of the dying trees.
"During regular maintenance we noticed some of our large trees were dying.
"It's extremely unusual to see pohutukawa of this age die naturally so we're sad to see them in this state.
"We found a granular substance sprinkled around the base of the trees so we sent it off for testing and it's been confirmed as Picloram, a high grade herbicide which we don't use in our parks."