A trainer banned over the Aquanita eight doping scandal denies giving horses "top-ups" on race days, evidence he says has not been influenced by "pretty graphic threats" he received.

Forced to take the witness stand as four other trainers fight their disqualifications, Trent Pennutto said he could not recall sending or receiving text messages referring to top-ups.

He conceded there was "a very small possibility" the texts referred to race day treatments using sodium bicarbonate, but argued the term could mean giving horses extra feed, water, exercise or shavings in their stable.

Eight people associated with thoroughbred management company Aquanita Racing were banned over the conspiracy to cheat using top-ups of sodium bicarbonate and Tripart paste in more than 100 races over seven years.

Pennuto denied being involved in the race day treatments despite exchanging text messages referring to top-ups with the float driver at the centre of Australia's biggest racing scandal, Greg Nelligan.

"I didn't know it was going on," he told the Victorian Civil and Administrative Tribunal on Wednesday.

"If it was happening it certainly wasn't happening to my knowledge or in front of me."

Racing Victoria barrister Jeff Gleeson QC asked Pennuto to name the person behind conduct detailed in a confidential document his lawyer submitted to the Racing Appeals and Disciplinary Board.

Pennuto refused, saying the person was associated with one of the parties involved in the Aquanita case.

"They were pretty graphic threats that were made," he said.

Pennuto said he took the matter seriously, adding he had a bad falling out with Nelligan when he left Aquanita.

But he said the matter did not affect his evidence to VCAT.

"No, there's been nothing to come of the threats," he said.

Pennuto said he never administered sodium bicarb with fellow banned trainer Tony Vasil or discussed doing so while working as his foreman.

Pennuto blamed desperation for his separate case involving stomach tubing one of his own horses on a race day last year, when he was caught with two syringes loaded with bicarb.

Much of the stewards' case against the Aquanita eight relied on 1000 text messages found on Nelligan's mobile after he was caught administering a paste to the Robert Smerdon-trained Lovani on Turnbull Stakes day in October 2017.

Like Pennuto and stablehand Daniel Garland, Nelligan and his wife Denise have been subpoenaed to give evidence to the VCAT appeal of trainers Smerdon, Vasil, Stuart Webb and Liam Birchley.

Gleeson said one of the most striking and disturbing instances of top-ups involved Birchley on Melbourne Cup day in 2015, pointing to texts between the Queensland trainer and Nelligan.

Nelligan claimed he had "two Cup horses as well" and texted: "Robert had me do one for the guy with the Cup horses a few years ago so it's not out of the circle of trust but I still don't tell him."

Gleeson said any suggestion the text references to top-ups meant giving horses food and water was fanciful.