Massey University volcanologist Shane Cronin says bigger eruptions from Mt Tongariro could present a significant agricultural hazard.
Tests of ash from the eruption of 6 August show moderate levels of soluble fluorine similar in range to the volcanic ash produced during the 1995 and 1996 eruptions of Mt Ruapehu.
But Professor Cronin says the ash fall from Tongariro was very thin and had a restricted distribution, so it posed no threat to farming beyond the immediate vicinity of the volcano.
He says, however, that if a larger eruption produced ash with similar concentrations of fluorine, it would be a hazard.
Professor Cronin says the risk of eruption is higher than usual so it is sensible to think about what precautions could be taken should future ash falls take place.
He says ash-covered pasture could force some livestock to stop grazing, leading to starvation and putting animals at risk of fluorosis disease.
Professor Cronin says farmers should move stock or provide alternative feed if ash falls are more than two millimetres deep or coat more than half of the pasture. Water troughs should also be cleaned out and refilled.