A vet who treated a Melbourne Cup hopeful with a banned substance in the lead-up to the race knew "full well" the horse was due to run, a Melbourne court has heard.

Amralah was a $9 favourite to win the 2015 Melbourne Cup when his owners withdrew him after a positive test result for a banned substance.

Four-time Cup winner Lloyd Williams and the other owners are suing a Ballarat vet and his clinic, saying their negligence caused them "catastrophic loss and harm".

A barrister for Williams, whose private horse racing operation Hudson Conway Racing paid STG550,000 ($A1.06 million) for Amralah, said his client should have been able to rely on the expertise of Brian Anderson.

"Anderson is not the local vet on the corner. He's a registered equine specialist," Mark Robins QC told the Victorian Supreme Court on Thursday.

The owners claim they missed out on millions in potential winnings, as well as prize money they had to give back because Amralah won a race with a banned substance in his system.

"When (Amralah) was withdrawn it was eight-to-one odds. We all know a 100-to-one outsider won," Mr Robins told the court.

He said Dr Anderson knew "full well" the horse was due to race.

The writ alleges Hudson Conway engaged the Ballarat Veterinary Practice to treat Amralah around September 2, at which point their stable vet made it clear Amralah was considered a potential Melbourne Cup winner.

On September 4, Dr Anderson injected Amralah with a long-acting cortisone known as Dexafort, which contains Dexamethasone, and advised it would take 14 days to leave Amralah's system, according to the writ.

On October 23, the South Australia deputy chairman of stewards advised Amralah had returned a positive test for Dexamethasone after a September 19 race, the writ says.

Andrew Broadfoot, acting for Dr Anderson, said the owners agreed to the injection and failed to show how the treatment was negligent.

"The fact that the horse tested positive subsequently does not in itself demonstrate that the selection of the drug was negligent," Mr Broadfoot told the court.

"What was it about the selection of this drug that led to it being unreasonable?

"What aspect of the advice was unreasonable?"

Mr Robins pointed to a notice issued by Racing Victoria on May 1 last year.

"It was in fact warning veterinary practitioners against the administration of Dexamethasone," Mr Robins said.

He said representations were made that the horse would be able to run 14 days after the treatment when the empirical reality is the horse was not able to race a month after.

A trial will likely be held in December.