Southern NSW trainer Doug Gorrel has been fined after one of his horses returned a positive to anabolic steroid Trendione in a post-race urine sample from a race in July.

The Wagga-based Gorrel was handed a $6000 fine by Racing NSW stewards for the positive test returned by Rita Red who had finished third in a 900m maiden at Corowa on July 24.

An investigation found that Rita Red had been treated with registered veterinary product ovu-mate, which contains altrenogest, every two to three days.

"Stewards were unable to conclusively determine the reasons for the prohibited substances being detected but, having regard to the evidence, were satisfied that such finding was not due to the administration of an anabolic steroid and was most likely due to the ingestion of an altrenogest containing substance," the stewards report read.

Rita Red was disqualified from her third placing.