Poultry owners and producers who fed birds with purchased organic grain or mash are urged to check yards and paddocks for parthenium weed following the discovery.

It is the first time the weed has been found in south-eastern NSW.

NSW Department of Primary Industries (DPI) State Priority Weeds coordinator, Phil Blackmore, said the Kiama detection is the 26th parthenium weed infestation to have been identified and managed in NSW since April 1 2020.

"No parthenium plants were found during an initial inspection of the Kiama chicken farm and the vigilant operators remained on the lookout for parthenium," Mr Blackmore said.

"The Illawarra District Weeds Authority was contacted by the owners when they found suspect plants which germinated in summer, just prior to Christmas, under favourable conditions."

Parthenium weed spreads rapidly, is dangerous to grazing animals and reduces crop and land values. Contact with the plant or pollen can cause serious allergic reactions in people.

"Six infestations are linked to contaminated organic whole grain chicken feed and mash and one is linked to contaminated conventional chicken feed," Mr Blackmore said.

"The contaminated feed infestations were traced to Queensland manufacturers who have worked with NSW DPI to eliminate parthenium seed from feed products.

"We have contacted suppliers, alerted buyers of potentially contaminated feed and conducted inspections of their properties."

NSW DPI urged everyone, particularly those who have fed poultry with purchased organic grain and mash to be on the lookout for parthenium.

Call the NSW DPI Biosecurity Helpline 1800 680 244 or local council with any suspected sightings of the weed.

Professional staff will help identify the plant and eradicate parthenium weed.

Check areas where hay, grain or seed has been fed to pets, livestock or chickens.

NSW has benefited from the program to keep the state parthenium-free since 1982, with the greatest number of parthenium infestations found and managed during the past 12 months.