However, in the early hours of Thursday, news came in that Caitlin Forrest had died from injuries she incurred in the four-horse fall.

McAuliffe said the racing industry in South Australia was in disbelief that one of it's most popular young riders had suffered such a "shock" fall.

After 30 years in racing, McAuliffe admitted it was the toughest day he had faced.

He feared Forrest was in real danger moments after a helicopter arrived to take her to an Adelaide hospital.

"The moment it happened was the most confronting thing in my broadcasting life," he said. "This beautiful girl with so much ability freakishly had her mount, Colla Voce, break a leg at the top of the straight, crashing to the ground with three other horses coming over the top of her.

"I knew, like so many of us on course, that Caitlin was in trouble and I had the terrible job of conveying messages to not only on-course spectators but also nationally, through my broadcasting commitments.

"They were mixed messages, some positive and some negative, but when I got the call very late on Wednesday night it seemed the horrific injuries had taken their toll."

Another jockey, Libby Hopwood, was also in an Adelaide hospital suffering concussion and a broken collarbone.

The other two jockeys involved, Adrian Patterson and Justin Potter, were able to ride in the final two races.

Last year Forrest was named the finest first-year apprentice rider in South Australia and secured the country premiership for winners.

Her mother Yvonne, a former jockey and now a trainer, was at Murray Bridge on Wednesday, while her father Darren is a leading harness racing driver.

"The whole racing world has come to a standstill," McAuliffe said. "Other people have told me when this happens at race meetings it will affect you for a long time.

"Underneath, I didn't want the last two races to go on. I sensed like so many that something was awfully wrong and our instincts were right. It's been 60 years, we think, since the last fatality in South Australia."

Leading Adelaide jockey Clare Lindop said the South Australian racing community was gathering around the Forrest family and that Victorian Jockeys Association chief executive Des O'Keeffe was flying into Adelaide on Thursday to offer support.

Former South Australian trainer David Hayes said the death had affected everyone. He said the trainers' hut at Flemington was a very quiet place on Thursday morning.

"You could sense that everyone was feeling this tragedy," he said. "I haven't seen the tape and I don't want to see the tape. It's one of the terrible aspects of a jockey's life and they deserve all the money that they are paid."