NSW racehorse trainer Stephen Farley has been banned for 12 months over a positive swab to cobalt.

Farley pleaded guilty to presenting the filly Di's Diamond to race with a prohibited substance in its system at Taree in February.

Di's Diamond finished sixth and tests on a pre-race urine sample returned a cobalt level of 377 micrograms and 380 micrograms.

The Racing NSW threshold for cobalt is 200 micrograms.

A Racing NSW stewards' inquiry on Tuesday heard that Farley has been a racehorse trainer for 17 years and he employs nine staff at his stable at Wyong on the NSW central coast.

Farley is best known as the trainer of Sincero, a high-class sprinter-miler who won the Stradbroke Handicap, George Main Stakes and Memsie Stakes during a career which netted 12 wins in 30 starts for more than $1.7 million in prize money.

His disqualification has been deferred for seven days, but he is not allowed to start a horse in any race during that period.

Farley has two acceptors at Gosford on Thursday while he has two horses nominated for the first day of the Scone Cup carnival on Friday.