Trainer Darren Smith has become the first person convicted of cobalt offences with NSW stewards finding him guilty of more than 40 charges.

Smith's defence that there was no threshold in place at the time his horses tested positive was rejected by the Racing NSW panel which relied on rules regarding the use of any substance which could alter a horse's blood picture.

Victorian trainers Peter Moody, Mark Kavanagh, Danny O'Brien and the Lee and Shannon Hope partnership are under investigation for exceeding the 200mcg/L threshold introduced under a local rule in April last year.

Smith faced 62 charges at a hearing last week relating to high levels of cobalt found in horses from his stable.

He admitted injecting horses with a substance obtained from disqualified harness racing trainer Shannon Wonson who he said described it as a blood booster.

Stewards issued charges relating to 21 horses under three different rules. After finding him guilty of two it was unnecessary to rule on the third.

The lowest reading from the horses involved was 224 with 12 reading above 1000, the highest being 6470.

NSW's chief steward Ray Murrihy said although it was not usual to publish detailed reasons, on this occasion the panel felt some explanation was warranted.

Stewards based their findings on three separate rules which define cobalt as a prohibited substance that (a) is an agent capable of causing both an action and effect on the blood system; (b) it is an haematopoietic agent and (c) is a hypoxia inducible.

Smith's lawyer Paul O'Sullivan submitted cobalt was not a prohibited substance before the threshold was introduced in January this year.

" ... the intent of the threshold is to recognise that cobalt naturally occurs within horses and elevated readings can result from horses being administered registered supplements in accordance with the manufacturers guidelines," a Racing NSW statement said.

"However, those issues do not arise in this case as Mr Smith has admitted that he was provided with a bottle by Mr Shannon Wonson that contained cobalt and he administered cobalt to horses in his stable.

"Mr Smith does not maintain that the elevated levels arose from administration of registered supplements in accordance with the manufacturers' guidelines but has instead defended the charges on the basis that cobalt was not a prohibited substance until a threshold was introduced.

"That construction is not supported by a proper analysis of the Australian Rules of Racing."

Stewards have refused nominations from the Smith stable since May and the trainer will learn his fate on March 20.