Vet Tom Brennan warned Racing Victoria's head veterinarian about cobalt being illegally used in the racing industry before being banned in both NSW and Victoria for the same thing, a tribunal has heard.

After hearing RV head vet Dr Brian Stewart and chief steward Terry Bailey talking on radio about cobalt, Brennan sent a text message to trainer Danny O'Brien saying "it's clear they haven't tested for it".

O'Brien responded with "dumb and dumber", the Victorian Civil and Administrative Tribunal heard on Tuesday.

O'Brien and fellow Victorian trainer Mark Kavanagh are trying to have their their respective four- and three-year disqualifications overturned by VCAT, arguing the laboratories that did the cobalt testing were not properly accredited at the time.

Their barrister Damian Sheales said before the text messages were sent in early 2014, Brennan told Stewart he was concerned about illegal cobalt use and Racing Victoria needed to be vigilant about it in the context of inorganic cobalt.

Brennan, who was outed by NSW stewards for six years, is no longer appealing his five-year ban in Victoria but will give evidence to VCAT.

The tribunal has heard Brennan eventually cracked and admitted Kavanagh and O'Brien paid him $3000 for three bottles labelled vitamin complex, after NSW stewards caught Kavanagh's trainer son Sam Kavanagh with a bottle containing massive quantities of cobalt hidden in his kitchen cupboard.

Sheales has said Brennan had always maintained he thought the bottles contained vitamin B12.

Flemington Equine Clinic equine nutritionist Samantha Potter said she saw Brennan put something else into a drip at O'Brien's Flemington stable, after drawing the substance from a bottle in his car fridge.

She agreed with Sheales that it was only after news about positive cobalt results that it took on "sinister connotations".

Potter said Brennan later asked her to post a vitamin complex bottle to Sam Kavanagh, telling her it was expensive and "don't let it break".

Potter said she recorded it in the clinic's postage book, which has since gone missing.