Co-trainers Terry and Karina O'Sullivan have been fined $3000 for presenting a horse to race at Sandown in September with a prohibited substance in its system.

Dexamethasone, a short-acting corticosteroid, was detected in a urine sample taken from the O'Sullivan-trained Dandy Gent after he finished second in a 2400m race on September 2.

The Racing Appeals and Disciplinary Board disqualified Dandy Gent from his minor placing.

The father and daughter training team are also facing a stewards' investigation into a high cobalt reading detected in a sample taken from Darragh at Ballarat in November.