Ash from this week's Mt Tongariro eruption poses no immediate threat to Wairarapa people or stock, initial analysis shows.
Massey University Professor Shane Cronin, of the university's Volcanic Risk Solutions Centre, said early tests on the ash found moderate levels of soluable fluorine. It was at a similar level to that produced by the Mt Ruapehu eruptions in 1995 and 1996, Mr Cronin said.
"Due to the restricted distribution and very thin ash fall, this currently poses no human health or agricultural threat beyond the immediate vicinity of the volcano," he said. Heavy rainfall since the eruption had removed much of the ash and its associated contaminants, he said.
The strong smell of sulphur many Wairarapa people reported was not surprising, he said, as sulphur dioxide was one of the chemicals commonly produced in a volcanic eruption.
Due to changing winds the smell had been reported as far north as Auckland and Mt Tongariro was still producing sulphur dioxide, so Wairarapa people would smell it for some time yet, depending on the wind direction.
Although there was no remaining danger from the eruption this week, Mr Cronin said Wairarapa farmers, along with others in the lower North Island, needed to be prepared in case of further, larger eruptions by Mt Tongariro with similar concentrations of fluorine.
Ashfall's biggest effect on pastoral grazing systems was covering pasture meant to eaten by grazing sheep, cattle and deer, while livestock drinking water in open troughs might also be contaminated, he said.
That meant stock could run the risk of starvation and dehydration, Mr Cronin said.
Deer were likely to be the most susceptible to fluorine, he said, followed by cattle. Sheep were likely to be the most resistant.
"Fluoride is adsorbed rapidly by grazing animals from ingested ash or contaminated water. In moderate levels of excess, it does not pass into milk," Mr Cronin said.
"Additionally, rural dwellers with roof-catchment drinking water sources should be vigilant to avoid ash runoff into water tanks. During and immediately following ashfall, the intake pipe to water tanks should be disconnected until ash has washed off the roof with rain."
Although ash was harmful to stock, Mr Cronin said in the longer term it contained chemicals that could be beneficial to the soil.