Scientists in the US have analysed Phar Lap's hair using a synchrotron, a light-emitting particle accelerator. A sample of Phar Lap's preserved skin was sent to a US laboratory in June for analysis.
The report said "The arsenic in the hair structure is consistent with a single large dose of arsenic between one to two days prior to death."
Forensic hair analysis expert Ivan Kempson said that while the quantity ingested was unknown there was "little doubt it caused his death".
However, racehorses in the era, including Phar lap, were commonly treated with the appetiser Fowler's solution, which contains arsenic. Some believe that the gradual buildup of arsenic from Fowler's solution and other tonics may have killed Phar Lap.
In 2000, Geoff Armstrong and Peter Thompson's book Phar Lap ghave the opinions of specialist veterinarians who studied eyewitness accounts and autopsy reports and came to the conclusion that Phar lap died of Duodenitis-Proximal jejunitis (also known as Anterior enteritis).
Armstrong said he was not convinced there was enough arsenic in Phar Lap's system to contribute to his death. A post-mortem at the time, he said, showed no large quantities of arsenic.
Phar Lap won 37 of 51 races in his four-year career including the 1930 Melbourne Cup. He was born in the Timaru area and sold as a yearling to Australia. He was taken to North America in 1932, where he won the world's richest race. He died in California soon after.