Trainer Darren Smith has lodged an appeal against a 15-year disqualification for cobalt-related offences which would effectively end his career.

At Friday's penalty hearing, Smith's lawyer Paul O'Sullivan said the trainer could not have known in April last year cobalt was a prohibited substance because there was no threshold.

Racing NSW stewards relied on the rule banning a wide range of products including those which can alter a horse's blood pattern and stimulate the production of EPO.

In Victoria, trainers Peter Moody, Mark Kavanagh, Danny O'Brien and the partnership of Lee and Shannon Hope are under investigation for breaching the 200mcg per litre of urine threshold introduced last year and adopted nationally from January 1.

Smith admitted injecting multiple horses in his stable with a substance he obtained in an unmarked bottle from disqualified harness racing trainer Shannon Wonson who described it as a blood booster.

The investigation began in May with nominations from the Smith stable refused until all horses returned clear tests.

Smith however, has not trained since and O'Sullivan argued the penalty should be for time already served and the cases involving raceday tests on 20 horses and one out of competition sample should be considered as one with no regard to Smith's previous record.

"The circumstances are extraordinary," O'Sullivan said.

"Fundamental to the case is that Mr Smith in April 2014 could not have known cobalt was a prohibited substance.

"At the relevant time the word cobalt did not appear in the rules.

"The facts are not in dispute that Mr Smith injected the horses with a substance. If he had been given a bottle labelled cobalt would he have known it was prohibited? The answer is no."

Stewards did in fact take into account it was Smith's 10th-drug related offence and judged each sample individually coming up with a total penalty of 15 years backdated to start on May 30 last year.

Smith received concurrent disqualifications of 13 years and 10 years under two separate rules regarding the administration of illegal substances and presenting horses to race with banned substances in their systems.

He received another two-year and one-year ban relating to an out-of-competition test, also to be served concurrently bringing the total to 15 years.

The horses involved in the 20 races, including 11 winners, have been disqualified.