Multiple Group One-winning trainer Robert Smerdon was in the habit of cheating amid a widespread practice of giving horses covert race day treatments, an inquiry has heard.

Racing Victoria stewards allege horses were given "top-ups" of bicarbonate over seven years, charging five trainers and three stablehands connected to management company Aquanita Racing at Caulfield.

Smerdon and his former stablehand and float driver Greg Nelligan were allegedly involved in more than 100 race day treatments between 2010 and 2017.

Stewards' barrister Jeff Gleeson QC said it was an extremely widespread and long-standing practice.

"The practice of top-ups among those eight people was knowing, it was brazen and it was systemic," Gleeson told the Racing Appeals and Disciplinary Board on Monday.

"It was, at least for Smerdon and Nelligan, a habit of cheating."

Stewards allegedly caught Nelligan administering a substance to the Smerdon-trained Lovani, who was then withdrawn from a race at Flemington on October 7 last year.

The inquiry was shown a video of stewards confronting Nelligan after he removed a yellow bag from under his jacket, took out a syringe and put it into Lovani's mouth.

"You can imagine the surprise and dismay of Nelligan upon realising that he'd been observed," Gleeson said.

"He, in a forlorn attempt to cover the matter up, attempted to conceal the plunger and the bag under his clothing."

The video showed Nelligan telling stewards the substance was "something I made up" and "no one else had anything to do with it".

Gleeson said there was a disturbing and telling awareness between some of the charged people about what stewards were doing.

"Surveillance of stewards and their activities was a matter of considerable interest to some of the charged persons showing a consciousness of guilt and a desire to avoid being detected."

Much of the stewards' case is based on seven years of text messages on Nelligan's mobile phone.

The tribunal heard those involved claimed the references to "top-ups" actually referred to topping up feed or water for the horses, which Gleeson said was simply implausible.

The text messages include references to the Melbourne Cup, although details about the horses concerned were not revealed.

Gleeson read texts between trainer Liam Birchley and Nelligan on Cup eve in 2015, during which the float driver noted: "got two cup horses as well. Don't tell Robert."

Nelligan later adds: "Robert had me do one for the guy with the cup horses a couple of years ago so it's not out of the circle of trust but I still don't tell him."

The tribunal heard Nelligan's wife Denise, also a registered stablehand, made "reluctant but damning concessions" to stewards.

The five trainers - Smerdon, Birchley, Stuart Webb, Tony Vasil and Trent Pennuto - and three stable employees - the Nelligans and Daniel Garland - were charged under an Australian racing rule dealing with dishonest, corrupt or fraudulent, improper or dishonourable action.

Smerdon, who handed in his trainer's licence, and the Nelligans are not participating in the inquiry, although the trainer has legal representation.

Only Queensland trainer Birchley, who has pleaded not guilty, attended the hearing on Monday.

Bicarbonate is used to help prevent the build-up of lactic acid but cannot be administered within one clear day of a horse racing.