Favourite Admire Rakti collapsed and died after the Melbourne Cup, adding a sad postscript to Australia's greatest race.

As the connections of Protectionist celebrated Germany's first victory in the Cup, Admire Rakti trailed in more than 200 metres behind him, clearly distressed.

The Japanese stallion was trying to match a modern weight-carrying record of 58.5kg in one of the fastest Melbourne Cups ever run.

Sent out as a strong $5.50 favourite, Admire Rakti vied for the lead with My Ambivalent for much of the race.

Jockey Zac Purton, triumphant aboard him in the Caulfield Cup last month, was convinced he was going to repeat that performance.

"The whole way through the run I thought I'd win. I didn't think they'd beat me from where I was," he said.

But with 800 metres to go the horse was not responding and Purton began to have suspicions all was not well.

"I didn't know it was as bad as it was," he said.

As they approached the home turn Admire Rakti was clearly under pressure and then dropped sharply back through the field.

Purton eased him down and virtually walked him over the line, 25 lengths behind the second-last horse, 200-1 outsider Mr O'Ceirin.

He dismounted and as Admire Rakti reached the tie-up stalls he collapsed. Vets pronounced him dead soon after.

"I'm just shattered," Purton said.

"He gave me such a great thrill there at Caulfield. They don't deserve this."

He said the owner Riichi Kondo was also very distraught.

Racing Victoria's Head of Veterinary and Equine Welfare Brian Stewart said Admire Rakti suffered sudden death syndrome which occurs in just 0.007 per cent of racehorses.

"It's usually one of two things, either through heart rhythm or ruptured blood vessels in that they bleed out internally," he said.

"We won't know which until the post mortem is done."

He said an irregular heartbeat which progresses to a condition called ventricular fibrillation, which means the heart beats in a bad rhythm, is similar to a heart attack.

He said ventricular fibrillation of the heart in racehorses can be stress related and adrenaline can play a roll.

He also said the death could have been caused by a by a large blood vessel like the aorta or a major abdominal vessel that ruptures and then bleeds out internally.

Dr Stewart said he will probably have some indication on Wednesday as to the cause of death but the full post mortem report will take longer.

It is the second consecutive year in which a horse has died in the Cup.

French mare Verema was put down after snapping a canon bone during the 2013 race.

In another post-race incident on Tuesday, seventh-placed Araldo kicked out at a fence on his way back to the mounting yard and injured a hind leg.

He was taken to a veterinary hospital for further treatment.

Early reports said the injury was serious.

The RSPCA said Admire Rakti's death was a stark reminder of the risks to racehorses.

"Sadly, injury and death are the price some horses pay for our entertainment in a sport that puts intense pressure on animals to perform to the limits of their endurance."

Coalition for the Protection of Racehorses spokesman Ward Young said about 125 horses have died during or shortly after Australian races in the past year.

"These events are more common then we would like to believe," he told AAP.