Farmers in the North Island are being warned to take precautions against the livestock disease facial eczema, which can rear its head after extremely dry pasture receives rain.
The fungus, which produces the toxic spores, thrives on drought desiccated pastures where the surface soil temperatures remain warm following rain.
Susceptible animals can suffer severe liver damage.
Asure Quality's facial eczema monitoring co-ordinator, Leo Cooney said any area receiving rain in the North Island, could see a spike in spore numbers in the next week to ten days.
"Facial eczema always spikes after a long dry spell and the east coast of the North Island and Taranaki, Whanganui and Manawatu have experienced that in the last month.
"And yesterday the east coast received quite a bit of rain, especially in Gisborne, where I understand they had about 45 millimetres and this - although it's very welcome rain and much needed - could spark the spore counts and cause a spike next week, so farmers are advised to be forewarned and forearmed."
Mr Cooney said facial eczema is rare in the South Island because it has much cooler night temperatures, and it only occurs in the Tasman area.