A post-mortem examination has determined Melbourne Cup favourite Admire Rakti died from acute heart failure and had no prohibited substances in his system.

The Japanese Caulfield Cup winner finished a distant last in the Cup on November 4 and collapsed and died after he was taken back to his race day stall.

Racing Victoria said it was satisfied based on the post-mortem performed by the University of Melbourne and expert analysis of samples taken from the horse, his death was due to "acute cardiac failure as a result of a cardiac arrhythmia that can be attributed to natural causes".

RV said the delay in confirming the cause of death was because of exhaustive tests on a kidney which showed "uncommon changes".

"The post-mortem examination identified uncommon changes in the horse's kidney that could possibly have been the result of a lack of oxygen prior to death, exposure to toxins prior to death or very rapid onset of autolysis (natural decomposition) just after the time of death," RV said in a statement.

"RV subsequently sought specialist analytical testing to eliminate the presence of substances toxic to the kidney in the samples, and obtained further expert pathological opinions on the nature and possible causes of the death."

Based on the analytical testing and expert opinions obtained, the changes seen in Admire Rakti's kidney could be attributed to natural causes," RV said.

Stewards found all Admire Rakti's veterinary and treatment records to be in order and there were no pre-race health concerns.

"Extensive analytical testing has been conducted on pre and post-race samples taken from Admire Rakti on the day of the Melbourne Cup and no prohibited substances have been found," chief steward Terry Bailey said.

"We are satisfied that the horse was presented in a fit state free of any substances that could have contributed to its death and unfortunately it has suffered a rare sudden death due to heart failure."

Admire Rakti was one of two horses to die after the Cup with Araldo suffering a leg injury when he took fright as he was coming back in front of the crowd.

He was later euthanised when scans showed his pastern to be shattered beyond repair.