In light of the horses that have died over east I thought I would post some research that I have done on what plants are toxic to horses, as it looks like these horses where killed as the result of Cyanide Poisoning, from the Aus Native wild peach, other wise known as the Quandong.
Cyanide poisoning results in hypoxia (deficiency of oxygen reaching the tissues). The first symptoms appear within a few minutes following consumption of plant material. Affected animals exhibit excitement, incoordination, convulsions, rapid and labored breathing, bloating, and coma. Death can occur in less than an hour due to internal asphyxiation
General: remove from source
Specific: sodium nitrite at 10 to 20 mg/kg with 500 mg/kg sodium thiosulfate as needed. The treatment is directed at breaking the cyanide ? cytochrome bond with the nitrite forming methemoglobin. Methemoglobin has a greater affinity for cyanide than does cytochrome oxidase, so it strips cyanide from the enzyme. The thiosulfate then reacts with the cyanide via the enzyme rhodanase forming thiocyanate which is readily excreted in the urine.
But as there usually dead with in the hour of eating the stuff, saving them is near impossible
This article is not Aus based but still interesting to read Prussic Acid Poisoning
This is probably the best guide I have found..
You can also buy a hard copy of this https://rirdc.infoservices.com.au/items/06-048
General Garden plants that are toxic, and not just to horses
A Data base for toxic weeds that horse links for Cats, Dogs, Horses, Humans,
Pigs, Poultry, Ruminants Australian Weeds and Livestock: Data
Weed Identification tool, this is fantastic Weeds Australia - Weed Identification