The careers of two trainers have been decimated by their cobalt disqualifications and ongoing appeals, a hearing has heard.

Trainers Danny O'Brien and Mark Kavanagh, who continue to train on stays of proceedings, are appealing their respective four and three year cobalt convictions to the Victorian Civil and Administrative Tribunal.

O'Brien, who took the stand on Monday to give evidence, said the number of horses on his books had fallen from between 180 and 200 in 2014 to between 70 and 75 after 18 months of the cobalt saga.

He said to have a horse trained in his stable costs around $60,000 per year, excluding vet fees.

O'Brien told VCAT president Justice Greg Garde that before his cobalt penalty he had 40 horses in full time work at his Flemington stable with another 50 at his Barwon Heads property.

He said there was a constant interchanging of horses to keep a full stable but from a staff of 50 in 2014 he now employed 30.

O'Brien said his Barwon Heads property was a $7 or $8 million investment from the greenfields to what it is today.

He explained his business model was reliant on training fees in 2010 but developing young horses that were shipped off to stud, and with shares in stallions, only 50 per cent of his income came from training in 2014.

O'Brien told the hearing that he had trained 16 Group One winners with a further 70 at black type level and had earned prize money of around $50 million from almost 1000 winners.

The trainer's barrister Damian Sheales told the tribunal the costs of appealing the cobalt penalties, which has now had 17 sitting days, had been a major burden to his clients.

"My clients spent $35,000 last week cross examining Adam Matthews," Sheales said.

Sheales had earlier told the hearing the number of horses on Kavanagh's books had fallen from 125 in 2014 to currently number 25.

O'Brien will continue his evidence on Tuesday.