Hundreds of sheep in the central west of NSW have died after eating a poisonous plant and bashing their heads open "like heroin addicts".

Plague proportions of darling pea have dealt another blow to farmers who suffered the impacts of the bushfire in Coonabarabran last January.

Farmer Stephen Knight, from Tannabah, Coonabarabran, with some of his Merino wethers that have been eating darling pea. Credit:The Land

The endangered native pea, which usually exists in the area in small quantities, has spread wildly following the bushfire, which burnt 54,000 hectares of the Warrumbungle National Park and adjoining farms.

Stephen and Louise Knight have lost 800 sheep to the noxious plant so far on their steep, rocky property, Tannabah.