NSW trainers and industry participants have again been warned about the substance synephrine found in orange peel and used in a number of herbal preparations and nutritional supplements for humans.

Racing NSW stewards have alerted trainers it can also be found in certain types of hay and grass.

Stewards have reiterated synephrine is considered to be a prohibited substance under the Australian Rules of Racing because it is an "adrenergic stimulant" known to affect the metabolism of body fat, and has been demonstrated to cause an increase in blood pressure in human subjects when taken orally.

They say synephrine is traditionally found in the peel extract of bitter orange (also called Seville orange).

"Bitter orange is an ingredient in a number of herbal preparations and nutritional supplements which are promoted to aid in weight loss and to suppress appetite in humans," stewards said in a statement.

"These products are available over-the-counter in pharmacies and in health food and dietary supplement stores, as well as from internet-based suppliers.

"However, synephrine has also been detected in certain plants, including the plant known as the common rush (Juncus usitatus), also known as pin rush or mat rush, in Mullumbimby couch, as well as in the leaves of the mandarin tree.

"Trainers are advised to ensure their horses do not have access to the common rush, Mullumbimby couch or the leaves of citrus trees if they are allowed access to open yards or are grazing on pasture.

"More recently, analysis of Teff Grass hay performed by the Australian Racing Forensic Laboratory has detected synephrine at high levels and accordingly trainers are warned not to feed this hay to horses in training.

"The ingestion of these materials listed above may lead to the detection of the prohibited substance synephrine in a race day sample."