The evidence of a vet who gave a Melbourne Cup-winning trainer's horses cobalt is the key to deciding whether two trainers will get their bans overturned.

Trainers Mark Kavanagh and Danny O'Brien are fighting their disqualifications in a Victorian tribunal after five horses returned positive results for cobalt.

Dr Tom Brennan originally kept silent about giving the horses a substance called vitamin complex in drips in their stables.

He changed his evidence when a bottle labelled vitamin complex was found in Kavanagh's trainer-son Sam's possession in Sydney.

Sam Kavanagh, who is appealing his disqualification in NSW, named Brennan as the source of the bottle.

The substance was analysed and found to contain high levels of cobalt but Brennan denied knowing that was a component of the product.

Brennan later told the Victorian Civil and Administrative Tribunal that Kavanagh and O'Brien knew he was adding the vitamin complex to drips and they each paid $3000 for three bottles of it.

The pair deny it, and their lawyer Damian Sheales questioned Brennan's truthfulness while saying he was central to the case.

"We know that Brennan is avaricious," Sheales said on Wednesday.

Racing Victoria's barrister Jeff Gleeson QC agreed that Brennan had given untrustworthy evidence before, but urged Judge Greg Garde to look at the surrounding facts.

"You still wouldn't know about (the cobalt drips) but for the fact that Brennan gave it up," Gleeson said.

Gleeson also said Kavanagh and O'Brien had lied about their knowledge of the cobalt source.

"Maintaining a consistent and plausible lie about these events is proving quite beyond O'Brien and Kavanagh," he said.

Kavanagh was disqualified for three years and O'Brien for four years, but they continue to train under a stay of proceedings pending the outcome of the VCAT appeal.

Kavanagh's horse Shocking won the Melbourne Cup in 2009.

After 23 days of evidence, the final closing submissions will be heard on Friday.