The trainer of retired sprinter Black Caviar is under investigation and could face a three-year ban after one of his horses failed a drug test.
Australian Peter Moody is "devastated" after Lidari tested positive for an elevated cobalt level following a race at Flemington, Melbourne, in October.
Mare Black Caviar enjoyed a record 25 consecutive wins, including the 2012 Diamond Jubilee Stakes at Royal Ascot.
"I've no knowledge or understanding as to how this could occur," said Moody.
"I take great pride in my role in thoroughbred racing in Victoria and strive to manage my business and support my wonderful group of owners and staff with the highest levels of honesty and professionalism.
"I will continue to pursue this goal over the next few months and do everything possible to clear my name."
Black Caviar retired in 2013 as the top-rated racehorse in the world, having amassed 15 Group One victories.
Racing Victoria Stewards said Lidari's urine sample contained cobalt at a concentration in excess of 200 micrograms per litre.
Cobalt is said to have similar effects to the blood-boosting hormone EPO.