The excitement of his first Melbourne Cup day led an otherwise experienced stablehand to experience a "brain snap" and breach the rules of racing by administering a treatment to the Gai Waterhouse-trained Tres Blue on the morning of the race.

The Racing Appeals and Disciplinary (RAD) Board heard stablehand Ryan Prendergast had been treating Waterhouse's second-string Cup runner in the lead-up to Cup day for a condition known as greasy heel.

He used an antibiotic cream three or four times a day on the imported horse which was stabled at the Werribee quarantine centre.

Waterhouse won the Melbourne Cup with Fiorente.

While he was aware of the rule that forbids any type of treatment to a horse during the 24 hours before it races, on Cup day Mr Prendergast performed his regular routine in Waterhouse's absence.

His diligence could have cost Waterhouse, who pleaded guilty, a six-month disqualification, but the RAD Board accepted the special circumstances surrounding the incident and fined her $2000 on a charge of negligence.

Board chairman Judge Russell Lewis said while trainers were "vicariously liable" for the conduct of their staff, the case was at the lower end of the scale.

"It was clearly an aberration on the part of Mr Prendergast," Judge Lewis said.

"But ultimately responsibility rests on the trainer."

The inquiry heard Waterhouse held weekly foremen's meetings attended by Mr Prendergast and at which subjects such as raceday treatment were discussed.

Counsel for the stewards told the board Waterhouse would have been allowed to use the cream Flamazine, which is not prohibited, had she asked permission.

Board member Darren McGee questioned the submission that Mr Prendergast had been affected by the magnitude of the day.

"It's hard to imagine a bigger brain snap," he said.

French trainer Mikel Delzangles was also fined $2000 last week for a similar offence involving his 2011 Cup winner Dunaden.

Neither Waterhouse nor Mr Prendergast attended the inquiry.