The herbicide picked for spraying at Coniston Beach and Hill 60 is not RoundUp but Brush-Off, which comes with a data sheet warning it is highly toxic to marine life.
The Illawarra Shoalhaven Joint Organisation (ISJO) clarified this while announcing a "full review" of its use of herbicides, after a public uproar followed news it was planning aerial spraying.
RoundUp faces fresh scrutiny after US courts found it caused several people's cancers.
Brush-Off, with the active ingredient Metsulfuron methyl, has a material safety data sheet (MSDS) which warns it is "very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment".
The spraying was intended to combat bitou bush.
ISJO CEO Leanne Taylor said Brush-Off was far less toxic when it was diluted.
"The information detailed in the MSDS applies to the concentrated product in the container, not the diluted product which is what the end user or applicator is dealing with when applying the chemical according to label direction," she said.
"Under normal operating procedures when these concentrates are diluted with water (as per label directions) the toxicity will decrease proportionately.
"Brush-Off when used according to label directions has been determined by Australian regulatory authorities to be in the lowest toxicity category of chemicals - unscheduled. It is quite safe to animals, insects and people."
Ms Taylor said the Australian Pesticides and Veterinary Medicines Authority had approved Brush-Off to fight various aquatic weeds, showing it was safe for water use.