Racing authorities have warned racehorse trainers to be alert after a confirmed case of the Hendra virus in the NSW Hunter Valley.

Racing NSW confirmed it had been advised of the isolated case of a 25-year-old unvaccinated mare on a property near Scone.

"At this time, advice is that there appears to have been no contact with thoroughbred populations," Racing.NSW said.

Scone is in the heartland of the NSW thoroughbred breeding industry with the Hunter Valley home to the biggest studs in Australia including Coolmore, Darley and Arrowfield.

Until now, Kempsey on the NSW Mid North Coast is the southernmost place in NSW to have a confirmed case of Hendra which first surfaced in Queensland in the early 1990s.

In 1994, Queensland trainer Vic Rail and 13 of his horses died from the virus.

Racing NSW chief veterinary officer Toby Koenig has posted guidelines and reminded racing participants of precautions on the administrative body's website.

siad.