Fears that illegal tree poisoning may have contaminated soil around a Lake Illawarra foreshore children's playground have been laid to rest by Shellharbour City Council.
Council staff say up to 200 trees between Oak Flats and Boonerah Point have been sliced and poisoned in recent years.
The latest incident occurred close to a children's playground at Boonerah Point with council forced to carry out soil testing to rule out a potential health hazard.
Chemical analysis of the soil showed that the dangerous amounts of poisons found in the native trees had not leached into the ground.
Council officers are now monitoring the affected trees and are continuing their investigations to determine what poisons have been used.
"Generally, commonly used herbicides impact vegetation at the time of application and are highly mobile, neutralising fairly quickly - making them difficult to detect," council environmental officer Andrew Williams said.
The council has stepped up its investigations into the poisonings and has doorknocked some properties.
Staff have also identified several mangroves that have been poisoned on the mudflats near Boonerah Point.
Shellharbour Mayor Marianne Saliba said the trees were vital to stabilising the banks of the lake, providing habitat to fish stocks and filtering the water.
"The dead and dying trees will remain standing for as long as possible," she said.
"Council will use all investigative tools and powers to stop this vandalism."
Cr Saliba said the council was considering planting more trees and placing signage at the site.
The dying trees will remain fenced off from the public to reduce the potential harm they could cause the community.