His heart is back in NZ I believe. They did an autopsy in 2000 that revealed bacteria that was only discovered in the 80's, so no one knew it was there in the 30's. This led to them saying the most likely cause of death was colic.
In 2008 they revealed that Phar Lap had ingested a massive dose of arsenic 30 to 40 hours before his death. However, in the 30's, arsenic was a common tonic given to race horses, and thus is not unusual in itself.