The science is "overwhelming" that cobalt can act as a doping agent and is performance enhancing for racehorses, Racing Victoria's head vet maintains.

Dr Brian Stewart, the driver behind Victoria's cobalt rule, has not changed his view that cobalt administered under the right circumstances could act as a doping agent in horses.

"The fundamental science is overwhelming," Stewart told a cobalt appeal.

Trainers Danny O'Brien and Mark Kavanagh's barrister Damian Sheales said there was "not one shred of science" to support Stewart's theory that cobalt was performance enhancing, but the vet disagreed.

"There's a mass of scientific evidence that would support its potential to have that effect," Stewart said on Thursday.

Before Racing Victoria introduced its cobalt threshold in April 2014, Stewart told the regulator's board cobalt salts had the effect of a doping agent in horses.

Stewart said that had been unquestionably proven in animal experimentation studies, although there were no equine-specific studies.

"My opinion was under the right circumstances it would act as a doping agent," Stewart told the Victorian Civil and Administrative Tribunal.

"In specific quantities at the correct time at the right frequency of administration, it is my opinion that it would act as a doping agent."

O'Brien, Kavanagh and Lee and Shannon Hope's appeal against their cobalt disqualifications heard a Racing NSW lawyer had said the regulator no longer maintained that cobalt enhanced performance.

Stewart said that may be a legal proposition but he and his counterpart at Racing NSW, Dr Craig Suann, still believed the science showed it was performance enhancing.

"The science was still held by Craig and I and the rest of the scientific community that cobalt is a very significant potential performance enhancer."

Stewart said the Victorian rule was brought in, ahead of the national threshold introduced in January 2015, to deter people from experimenting with cobalt.

The regulator had a provision to possibly do out-of-competition testing.

"The cobalt that will be used in out of competition won't be administered close to the time of racing, so only the very stupid or very arrogant trainers would get caught on raceday," he said.

Stewart admitted the regulator made a mistake in not warning the industry in 2014 that commonly-used products such as injectable vitamin supplement VAM could push cobalt levels over the threshold.

Asked if there should have been warnings, he said: "With the benefit of hindsight, yes."

There were warnings about such products when the national threshold was halved, to 100 micrograms per litre of urine, from September 1 this year.