A surge in hay-making as farmers seek to make the most of excess grass growth has increased the risk of livestock developing facial eczema later this summer.
But so far unseasonal wet weather has kept the rate of infection lower than the same time last year.
The fungal disease affects mainly North Island farms in late summer and autumn, as the spores that spread it to livestock multiply in the base of dried-out pasture.
Infected animals suffer liver and skin damage which in severe cases can kill them.
Facial eczema is estimated to cost the farming sector millions of dollars a year in lost production and treatment.
Leo Cooney of Assure Quality, which monitors the facial eczema risk, says the grass boom in spring has led to an increase in hay production.
He says topped paddocks can increase the spread of the disease, because it creates litter and when rain falls, the litter starts to rot which is a nutrient for the fungi to survive on and produce toxic spores.
Mr Cooney says though it's early days, farmers should take precautions by spraying pasture with fungicide or adding zinc sulphate to stock water troughs.