Champion racehorse Black Caviar has died a day before her 18th birthday.

A statement on Saturday on behalf of Black Caviar's owners announced the death.

The Australian star captivated the nation and retired in 2013 with 25 wins from 25 starts, taking $7,953,936 in prize money.

"She was a remarkable horse who gave us all the ride of a lifetime and wonderful memories," trainer Peter Moody said in the statement.

Moody later revealed the cause of death was laminitis, a painful disease of the foot that eventually progresses to a point that a horse can no longer walk.

"She had a milk infection about a week ago and we just treated it like you do with all broodmares, but like a lot of treatments, it went straight to her feet," Moody told racing.com while at Caulfield today.

"Basically, it killed her feet.

"She had the foal this morning, a colt foal by Snitzel, and they put her down shortly after that on humane grounds.

"They scanned her feet yesterday and there was no blood flow at all to her feet. There was minimum blood flow to her feet anyhow but there was none and she's such a big girl.

"I rang all the staff and told them. They were all very upset."

Moody said he cried in the car for an hour after he got the news.

"You don't think it affects you," he said.

"But you are being stupid to think it doesn't. It's impossible not to get attached to most animals, let alone one like her."

Victoria Racing Club chairman Neil Wilson extended his condolences on behalf of the club.

"Today's news is a shock to us all. Black Caviar was more than a champion racehorse, she was an important part of life for many people," Wilson said.

"Her brilliance, her presence and the pleasure and inspiration she gave to so many was a special moment in time for horse racing and sport more generally.

"She has created a piece of history that will be remembered forever, and it was a privilege for all who saw this wonderful equine athlete in action."

Trainer Mick Price, a close friend of Moody, was there at the beginning of Black Caviar's journey.

"I stood next to Peter Moody at Melbourne Premier when he bought her," he told ABC Melbourne.

"I remember the day, and I remember her first start, and every start thereafter."

Price said Black Caviar's achievements would stand the test of time.

"I think in our time, or certainly in my time, I will not see a sprinting mare that is going to win 25 in a row," he said.

"She was trained perfectly, she raced perfectly, and 25 out of 25, it's something you only see in the story books."

Price said the impact Black Caviar had on racing in Australia could not be overstated.

"People love heroes in any sport, anywhere, anytime. They love the heroes and she was a hero of the sport," he said.

Racing Victoria interim chief executive Aaron Morrison said Black Caviar was "the people's champion".

"There is no doubt that her passing will touch many," he said.

"The night Melburnians packed out Federation Square to watch her conquer the world at Royal Ascot was testimony to her popularity. 

"Black Caviar will be forever remembered as a champion of the Australian turf. She is a horse that brought joy to so many."

Laminitis is the inflammation of the laminae of the foot, which is the soft tissue structures that attach the coffin bone to the hoof wall. 

The damage it causes leaves the horse in extreme pain, and once a horse has had an episode of laminitis they are susceptible to future episodes. Laminitis can be managed but not cured.

Laminitis can occur due to many reasons, including excessive weight bearing, concussion, fillies and mares coming into season, cold weather, infections and toxaemia, retained placenta, and drug inducement.