Racing NSW stewards have banned all nominations from Newcastle trainer Darren Smith after finding elevated cobalt chloride readings in samples taken from some of his horses.

Cobalt is found naturally in horses but elevated levels of the substance can increase the production of red blood cells in a similar manner to the banned drug EPO.

Smith had three runners at Saturday's Rosehill meeting but stewards ordered their withdrawal and rejected the trainer's nominations until an inquiry is held.

Chief steward Ray Murrihy told TVN he questioned Smith on Thursday after an unannounced inspection of the trainer's stable uncovered evidence of the administration of cobalt to horses.

Murrihy said urine samples taken from a number of Smith's horses had revealed high cobalt readings, leaving stewards with the job of retesting previous samples to determine the depth of the state's first thoroughbred case involving the substance.

"There has been some preliminary reports involving cobalt being found in samples," Murrihy said.

"It's at a very early stage of the inquiry. We need to have the samples confirmed."

Murrihy said stewards withdrew Smith's horses to "ensure the integrity of the races going forward is 100 per cent".

Harness Racing NSW has suspended a number of trainers for lengthy periods for producing horses on tracks with elevated cobalt readings in 2014.

US vets have expressed concern elevated levels of cobalt could cause heart, nerve or thyroid problems for horses.

Smith has attracted the attention of stewards on eight previous occasions for treatment-related offences.

He served an 18-month penalty for using trenbolone implants in 1999.