Jockey James McDonald was thrown off Cox Plate frontrunner Via Sistina in dramatic scenes during a trackwork session at Moonee Valley on Tuesday morning.

The incident happened in front of a large crowd, which was attending a breakfast function to celebrate the upcoming race on Saturday.

McDonald was unhurt in the incident but Yulong Investments racing manager Troy Stephens said Via Sistina — which had been listed as the second favourite to win the race — would be "very unlikely" to run after doing four laps of the track without the jockey.

"Very disappointing, we can see her walking back in now and she looks fine which is the main thing," Stephens told SEN Track.

"Just on straightening there she's just tripped over and dislodged James, and she's obviously done a bit of work after that.

"James said on straightening she's just tripped in front of him, and she was lucky she didn't trip [over] … she had that bandage wrapped around that hind leg, which was probably spooking her a lot too.

"She's a super mare. As long as she's all right, we're happy."

Via Sistina's trainer, Chris Waller, said the horse was uninjured.

"She doesn't have a hair missing, there is no blood, no skin off," Waller said.

"We've had our vet look over her, the Racing Victoria vet. I've been over her as well and I can't believe [she is OK].

"When I first saw it I feared the worst, something tragically might have gone wrong, and you take comfort that that hasn't happened and that she's safe."