An initial autopsy has revealed that Japanese stayer Admire Rakti died from a heart attack after the Melbourne Cup, reports the ABC.

Dr Chris Whitton, an equine veterinarian at the University of Melbourne's veterinary hospital in Werribee where Admire Rakti's post-mortem examination was conducted, said it may take weeks to establish exactly why the horse died.

"One of the problems we have is that in about half of the horses that die sudden deaths like that, even with a full post-mortem, we don't get an answer," Dr Whitton said.

"In that other half where we get an answer, some of them are due to haemorrhage into the lungs and some of them are due to heart failure, sometimes for reasons again that we don't really understand very well."

While haemorrhage into the lungs was a possible cause of death, Dr Whitton said it was quite common in racehorses.

"Haemorrhage into the lungs happens in nearly all horses that race to some degree," he said.

"It's usually a small amount and of no consequence. But in some horses it happens to a greater degree and very occasionally and very rarely so much that they actually die.

"And the reason it happens is because horses are pushing themselves to the limit of biology. They've got a very great need for oxygen to their muscles and to do that they have a very high blood pressure in their lungs. The high blood pressure means that there's a risk of those vessels bursting."

Dr Whitton said whips and heat were not likely to be factors, and that racehorses pushed themselves hard as a matter of instinct.

"A lot of it's instinct. These horses have a very heightened flight response," he said.

"When they go, they go fast and they run until they think they need to stop.

"There's risk involved in doing anything with horses because they're such a powerful animal, even them just being kept in a paddock, they'll run into fences, they'll fall over and break things.

"You could make the case that because they're so well managed in racing that you reduce some of that risk because they're kept well, but yes, you increase the risk of other things happening because of the racing.

"But we know the risks in racing are fairly low. The death rate is less than one in 2,000 starts."