Disqualified vet Tom Brennan says his misplaced trust in a colleague is the reason he and three trainers are fighting bans for cobalt use.

Brennan has been outed by both NSW and Victorian racing officials over high levels of cobalt found in horses from the Flemington stables of Mark Kavanagh and Danny O'Brien and in horses from Sam Kavanagh's Sydney stable.

Brennan, Sam Kavanagh, Flemington Equine practice manager Aaron Corby and Adam Matthews, who worked for Brennan, are all appearing before the Racing NSW Appeal Panel to fight their disqualifications in the Sam Kavanagh case.

In February last year, Sam Kavanagh told stewards Brennan was the source of a product called Vitamin Complex, later found to contain high levels of cobalt.

After initially denying Kavanagh's claims, Brennan told Racing NSW stewards it came from Matthews who told him he had a product he sourced from Canada which he had successfully used on harness racing horses.

"I expressly asked if it contained cobalt and he assured me it didn't," Brennan said.

"He gave me the names of a few trainers and said the horses had won with it and not produced any positives.

"That gave me an extra layer of confidence."

Brennan said Mark Kavanagh, Sam Kavanagh and O'Brien all paid $1000 per bottle of Vitamin Complex with all the money going to Matthews.

"There was no providence on the bottle and no instructions," Brennan said.

"I drew some out and it looked like B vitamins - it was orange.

"We were using it to aid recovery after races or hard gallops.

"It is the biggest mistake of my life."

After paying for three bottles, Brennan said Mark Kavanagh stopped using it after one, telling the vet it was "shit".

Mark Kavanagh and O'Brien are fighting disqualifications imposed in Victoria and continue to train on a stay of proceedings pending a VCAT hearing.

Sam Kavanagh is fighting a nine-year ban sparked by high levels of cobalt and the presence of caffeine in a post-race sample from Midsummer Sun after he won the Gosford Cup in January 2015.

Other horses in his stable also returned high cobalt levels.