Hunter Local Land Services regional weeds coordinator Matt Kennedy said it is illegal to own or deal with frogbit in NSW, as the plant can rapidly invade and smother waterways and is a serious biosecurity threat.

The plants float on the water surface, and have smooth, rounded, fleshy green leaves up to four centimetres wide, with sponge-like sections on their undersides.

A floating freshwater plant from Central and South America, frogbit has been kept and traded for use in fish ponds, aquariums and water features.

"Frogbit is illegally traded online. It is important to remember all aquatic plants should be purchased from a reputable source," Mr Kennedy said.

"There are heavy fines for offences committed under the Biosecurity Act 2015, however, the Hunter region's primary concern is to identify infestations, eliminate again and prevent this weed.

"Initial investigations of the infestation have shown the weed to be limited to the one dam. All landholders of the Hunter Region are asked to be on the look out for this weed and report it to their local biosecurity (weeds) officers.

"If you believe you have or have seen frogbit please get into contact with your local biosecurity (weeds) officer so we can once again eliminate this plant from our region.

"If you have frogbit do not just throw it away, please get into contact with the officer so they can advise on how to best destroy the plant."

Find your local weeds officer's contact details at hunterregionalweeds.net.au.