Veterinarian Tom Brennan fell out with Mark Kavanagh before the trainer had a horse return a positive test to cobalt, an inquiry has been told.

During the fourth day of the probe into cobalt positives returned by horses trained by Kavanagh and Danny O'Brien, a Racing Appeals and Disciplinary Board hearing was told the relationship between Brennan and Kavanagh soured over treatment and expenses for Australian Cup winner Super Cool.

The inquiry heard Super Cool had bleeding to a joint that needed attention.

Brennan said he provided correct veterinary care to Super Cool but Kavanagh continued to work the horse against the vet's advice.

After receiving a bill of $25,000 for the work carried out on Super Cool, Kavanagh cut ties with Brennan as his stable vet.

Brennan was still working for Kavanagh when Magicool received a drip that later led to the horse returning an elevated reading to cobalt after winning at Flemington in October last year.

Kavanagh and O'Brien have pleaded not guilty to their cobalt charges while Brennan has pleaded guilty to the majority of his charges except administration to affect the performance of the horses.

Barrister Damian Sheales, appearing for Kavanagh and O'Brien, was testing the credibility of Brennan who had earlier admitted lying during interviews with both Racing Victoria and Racing NSW stewards.

Sheales said Brennan had told countless lies during previous interviews to which Brennan counter-claimed: "The difference is I'm telling the truth and his clients are maintaining the lies."

Brennan is appealing a six-year disqualification imposed by NSW stewards in September.

Sheales asked Brennan if he had done a deal with Racing Victoria.

To clarify the matter, Jeff Gleeson QC, for Racing Victoria, said if Brennan hadn't fronted the inquiry he would have been looking at a maximum penalty.

Sheales suggested that Brennan disliked both Mark Kavanagh and his son Sam for the fallout from the cobalt saga that has seen Sam Kavanagh disqualified for more than nine years.

Brennan said Mark Kavanagh had manipulated his friendship with former Australian Turf Club employee Matt Rudolph.

"I do not hate the Kavanaghs," Brennan said.

"I've been extremely upset by the way Mark Kavanagh threw Matt Rudolph under the bus.

"I know he and his family have been under a lot of pressure."

The hearing continues on Friday.