Three leading Melbourne trainers say they are at a loss to explain how horses in their care could have tested positive to elevated levels of cobalt.

Over the past two days Peter Moody, Mark Kavanagh and Danny O'Brien have all issued statements after being told of the analyst's findings of cobalt in excess of the threshold 200 micrograms per litre of urine.

All three say they will co-operate with Racing Victoria stewards, saying cobalt is not given to horses in their stables.

The country-based father-son partnership of Lee and Shannon Hope is also the subject of an ongoing investigation after three horses in the stable returned elevated cobalt levels.

"I am devastated to be informed by Racing Victoria Stewards that one of my runners has tested positive to a banned substance during the 2014 spring carnival," Moody said.

"I have no knowledge or understanding as to how this could occur and will work with the Racing Victoria Integrity Services Department to bring this matter to a conclusion as soon as possible."

Kavanagh and O'Brien expressed similar sentiments in their statements with O'Brien saying it could not have happened through normal feeding and treatment practices.

"We are examining everything and our vet is going through everything he does," he told Racing.com.

"Obviously it is a cluster and maybe something will come to light."

RV's chief steward Terry Bailey says the investigations are ongoing and no time frame has been set for official hearings.

"It is now our priority to gather all the facts to determine the circumstances surrounding each case. We will not be putting a timeline on the completion of this process and will not be commenting on the specifics of each case whilst our investigations continue," Bailey said.

"Our investigations will determine whether any or all of the trainers will be charged with a breach of the Rules of Racing. They have the presumption of innocence and are free to continue racing at this time."

Cobalt is an essential mineral nutrient which affects multiple body systems. It occurs naturally and is an ingredient of feed and vitamin supplements but is toxic at high dosages which prompted the introduction of the threshold.

High levels of cobalt are believed to induce hypoxia, or a lack of oxygen, forcing the body to produce more erythropoietin (EPO).

Cobalt is a banned substance under the rules of the World Anti Doping Agency.

The threshold, which Bailey described as "generous" was introduced to racing last year.