Ten horses have died suddenly at properties in the Mornington Peninsula, southeast Melbourne and southwest Victoria since July 4.
All the horses are believed to have died quickly with short-term, non-specific signs of illness.
Agriculture Victoria has launched an investigation into the illnesses and deaths but the probe remains in its early stages.
"It is not yet known what has caused these deaths and to date common links are not evident," Victoria's chief veterinary officer Graeme Cooke said.
"Early testing for some known diseases in horses has been negative."
Multiple horse deaths on a single property are uncommon.
Investigations typically look at infectious causes and ingestion of toxins but it is not always possible to pin down a cause.
There have been reports on social media channels of further affected properties, with Agriculture Victoria calling on any owners who have not reported deaths to contact it or their veterinarian.
"We know this is a difficult time for horse owners, that's why we are working with veterinarians and a range of experts to find answers," Dr Cooke said.
"Please do not share unconfirmed information online or on social media."
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