It was discovered near Cessnock when a biosecurity weeds officer from Cessnock City Council noticed an unusual plant on a dam during high-risk pathways inspections.

Using the connections of the Hunter Weeds Technical Team, photos of the suspected prohibited matter were sent to MidCoast Council (which has controlled three frogbit infestations since 2017) for identification.

Hunter Local Land Services regional weeds coordinator Matt Kennedy said the weeds officer from MidCoast Council was able to identify the plant as frogbit based off the photos, so samples were then sent off for formal identification at the NSW Herbarium, who confirmed the outbreak.

"The Cessnock Weeds Officers didn't waste time inspecting nearby properties and utilised desktop surveillance with recent aerial imagery to highlight at risk properties," Mr Kennedy said.

"Three more dams across two more properties were found to also have outbreaks of this high risk weed and it was deemed specialised treatment of the sites were required, as well as booms and temporary sedimentation fencing were installed to limit any possible spread."

Aerial inspections of the area were undertaken, confirming the outbreak had not spread beyond the four dams. Spot checks along Black Creek have not shown any signs of frogbit establishing along its course.

From here Cessnock Council weeds officers will undertake follow-up inspections and further specialised treatment as required. Ongoing surveillance will be undertaken for at least the next three years, based on seed viability, to guarantee eradication of this highly invasive weed from the region.

"Neighbouring councils have been notified and their teams are on notice to keep a look out for this weed downstream and on the Hunter River, with some targeted inspections underway," Mr Kennedy said.

"We're asking the community to be vigilant as well; all residents of the Hunter region are asked to play their part in safeguarding our waterways."

Mr Kennedy said landholders shouldn't be concerned about reporting this weed, if they suspect it on their property. Funding for initial treatment will be provided to cover the costs of best practice control measures.

"It is not fully known how this and other infestations of frogbit keep popping up on private and public land across our region and in other parts of NSW," he said.

"Transplanting of the weed is known to occur and the illegal online trade of this weed species and other weed species which are a serious threat to our environment and production land are occurring through places like Facebook Marketplace cannot be ignored as one of the most likely sources of future infestations.

"When buying plants everyone should make sure they are purchasing from a reputable seller that list botanical name alongside a common name."

The community is encouraged to send through pictures of suspected frogbit to their local Biosecurity Weeds officer via hunterregionalweeds.net.au or through their local council.

Reports can also be made via the DPI 'Report a biosecurity concern' form online or call the Invasive Plants and Animals enquiries line on 1800 680 244.

For information on what plants should not and must not be traded in NSW, visit dpi.nsw.gov.au.