The vet at the centre of a cobalt inquiry was so convinced a product he supplied to Sam Kavanagh did not contain anything illegal, he approached Melbourne trainers Danny O'Brien and Mark Kavanagh about using it, a hearing has been told.

Dr Tom Brennan of the Flemington Equine Clinic has been charged by both Racing NSW and Racing Victoria stewards in cases involving illegal cobalt levels found in racehorses from three stables.

After Sam Kavanagh named Brennan as the source of Vitamin Complex, later found to have high levels of cobalt, he was charged in the cases against O'Brien and Mark Kavanagh, the father of Sam.

Chris Winneke, appearing for Brennan, told Racing NSW stewards on Thursday, the vet had been assured by Adam Matthews, a former junior partner at the clinic, it assisted horses in their recovery and was not illegal.

Brennan took his word and did not have the substance analysed.

"You should accept that Tom Brennan did not know. He was told by Adam Matthews it helped horses to recover after gallops," Winneke said.

"He says he approached Kavanagh senior and O'Brien about using it.

"No sane vet would not have tested it if he thought there was cobalt in it," Winneke said.

"It might be the stupidest thing he didn't do."

The inquiry was sparked after the Sam Kavanagh-trained Midsummer Sun returned a positive result to caffeine and high cobalt levels in a post-race swab after he won the Gosford Cup in January.

Former harness racing trainer Mitchell Butterfield has admitted to drenching Midsummer Sun on Gosford Cup day at the request of John Camilleri who Sam Kavanagh had arranged to treat the horse.

Butterfield also admitted injecting the horse with a syringe handed to him by stable foreman Michael O'Loughlin.

"It would have to be at least a strong possibility that substances from Butterfield and Camilleri are the source of cobalt and caffeine," Winneke said.

"You will never know what goes into this horse (Midsummer Sun) on the 9th."

Brennan originally told the inquiry the substance did not come from his practice, which he later retracted, leading to charges of giving false evidence in addition to supplying the liquid.

Sending the bottles of vitamin complex was "extraordinarily unwise"," Winneke said.

The inquiry has been adjourned for written submissions.