Part of the reason is believed to be a build-up of toxins in the plant over a long period of dry conditions in the lead-up to the autumn break.
Livestock Logic vet David Rendell, who is based in Hamilton, estimated "thousands" of sheep would have been lost due to the phalaris sudden death outbreak in recent weeks.
"We need to get more data on this so we can understand the factors influencing it," he said.
Producers who have introduced sheep on to phalaris after the break are being urged to complete a survey at www.livestocklogic.com.au
"We need to hear about those who have had problems and those who haven't to understand the factors involved," Dr Rendell said.
In one instance, a Hamilton-region producer lost 50 sheep within 36 hours of shifting from native green pasture to phalaris.
Risks of poisoning occurring appeared to be much higher following a long, dry summer or a late autumn break, or both Dr Rendell said.
"There appears to be the highest risk at four to eight weeks post-break," he said.
All strains of pasture appeared equally toxic.
And sheep, which are very selective grazers, were at higher risk than cattle. There was also no evidence that shifting an affected mob back off the phalaris pasture would then reduce the mortality rate, Dr Rendell said.
The problem was not evident in other parts of the state which had not suffered from seven months of dry conditions. A similar poisoning occurred after a prolonged dry spell in 2003-04.
Producers putting stock onto phalaris were advised to ensure sheep "had a full belly, and are put in the new paddock in the evening to reduce the amount they eat in their first few hours".
Livestock Health and Pest Authority district vet Tony Morton said there were similar reports in the Wagga Wagga and Albury areas.
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