The Veterinary Association has had no reports of farm stock in the National Park area being affected by ash from Mount Tongariro.
Tongariro erupted for about five minutes at about 1.30pm on Wednesday, sending a plume of ash and gas 4km into the air.
The eruption was smaller than the Tongariro eruption on 6 August, when the ash did not have any negative effect on farm pasture or stock.
The Veterinary Association says farmers need to be aware though that if Tongariro emits a significant quantity of ash, there could be animal welfare issues if stock don't have access to clean feed and water.
Dr Wayne Ricketts said ash can cause skin infections and damage an animal's lungs.
Ash settling on pasture can also wear down the teeth of grazing animals, especially cattle and horses.
Federated Farmers is advising its members in the central part of the North island, as well as Waikato, Bay of Plenty, Hawke's Bay and Tararua, to be on the alert if there's a significant eruption from either Mount Tongariro or Ruapehu.
Spokesperson Katie Milne said farmers should make sure they are well prepared.
She said the high level of acidity in ash and its abrasive nature when it falls on pasture, can cause flurosis in farm stock, with deer being the most susceptible.
