Southern NSW farmers have reported deaths in old sheep from the plant which appears to be prevalent this year.

Hume Livestock Health and Pest Authority (LHPA) district veterinarian Dr Tony Morton said hairy panic contained a liver toxin and young sheep were particularly susceptible.

The weed, which has been prevalent in the Riverina for 15 to 20 years, was particularly noticeable this year and Dr Morton said the conditions were perfect for it to thrive.

"Hairy panic loves bare paddocks and summer rain," he said.

In cases where stock deaths were reported the amount of hairy panic usually accounted for 50 per cent or more of the available feed.

Dr Morton said the initial symptoms, before the eventual photo sensitisation of an animal, included swollen eyelids, runny eyes, and a red or black nose.

However, he warned photo sensitisation could be caused by other factors besides hairy panic poisoning.

To prevent losses he said it was best to move the stock from affected paddocks and put them into a shearing shed or structure which provided good shade.

"Provide them with food and water, and hope the liver will recover," he said.

He advised farmers contact their veterinarian and also talk to their agronomist if they need advice about controlling hairy panic.