Farmers are being urged to look out for a poisonous weed blamed for killing about 100 sheep near Horsham.
Lower Norton grazier Kevin Wilkie says a quarter of the weaner lambs in his paddock died suddenly.
Animal health officer Kate Ferrier investigated the deaths and says it is an unusual case.
Ms Ferrier says there is a toxin in the weed, commonly called witch grass, that is relatively unknown.
She says the symptoms of poisoning are quite clear.
“It looks like extreme sunburn, the [animal's] face becomes quite swollen,” she said.
“You might see the ears are quite swollen and droopy, the eyes swollen, so it’s quite distinctive what happens.”