Vet Tom Brennan admits lying to protect himself and trainers Danny O'Brien and Mark Kavanagh but insists none of them knew a vitamin complex drip contained cobalt.

Brennan has accused Victorian racing authorities of pushing their own agenda about cobalt and claimed certificates showing illegal cobalt levels in the trainers' horses were doctored.

He said it was garbage to suggest he or the trainers knew the vitamin complex drips contained cobalt, but insisted O'Brien, Kavanagh and Kavanagh's son, Sydney trainer Sam Kavanagh, all paid $1000 a bottle.

"At no stage did we know that we were ever using cobalt," Brennan told O'Brien and Kavanagh's appeal over their cobalt disqualifications.

Brennan has again blamed former Flemington Equine Clinic vet Dr Adam Matthews for supplying the vitamin complex, which Matthews denies.

"Unfortunately I got hoodwinked by an associate of mine," Brennan told the Victorian Civil and Administrative Tribunal on Thursday.

"I trusted him and the trainers have trusted me and we all ended up in this mess.

"Yes I lied to protect my clients and I lied to protect myself and I apologise."

Brennan did not tell stewards about the vitamin complex until July last year.

He denied making up a story about Matthews sourcing it from Canada, although he now believed that was nonsense.

Brennan admitted he threw out a bottle of vitamin complex "in a moment of panic" after learning of the cobalt positives and destroyed a clinic record of him sending bottles to Sam Kavanagh in Sydney.

Brennan also told Sam Kavanagh to get rid of his bottle.

A vitamin complex bottle tested by NSW stewards was found to contain cobalt.

Racing Victoria barrister Jeff Gleeson QC suggested Brennan, O'Brien and Mark Kavanagh knew all along that it was cobalt, which the vet denied.

"No, that's garbage, absolute garbage," Brennan said.

"The only reason you guys are still trying to say we knew is because it suits your agenda. It's because you got up on the first day and made this big song and dance about it."

Brennan accused Victorian stewards of trying to pass off analysis certificates as being accredited by the National Association of Testing Authorities when they were not.

"We were presented with certificates to say that these horses had exceeded thresholds and they're doctored-up certificates.

"We've got stewards telling laboratories what needs to go on these certificates."

O'Brien and Kavanagh's barrister Damian Sheales cited an October 2014 email from Racing Analytical Services Laboratory to Perth-based ChemCentre asking it to issue amended certificates that were NATA-endorsed.

Gleeson said retests had recorded the same results as the initial tests.

Sheales said Brennan knew his clinic's liability under potential insurance claims would be reduced if the trainers knew and agreed to the treatment.

Brennan said he did not know about the insurance situation but "they knew and agreed".