Via Sistina has won the Cox Plate at Moonee Valley in a dominating performance, completing an incredible comeback after a mid-week trackwork accident nearly put her out of the race.

Jockey James McDonald rode Via Sistina from the middle of the pack, making his move towards the corner to run down Japanese horse Prognosis before racing away to clinch the Australasian weight-for-age championship.

The Chris Waller-trained Via Sistina won by an equal-record eight lengths, from Prognosis and Broadsiding.

The seven-year-old mare smashed Winx's Moonee Valley track record set in the 2017 Cox Plate, stopping the clock in 2 minutes, 1.07 seconds — 1.87 seconds faster than the four-time Cox Plate winner. 

An emotional Waller spoke after the race, revealing his worries when Via Sistina nearly came down during trackwork on Tuesday.

The horse was nearly out of the race, when McDonald hit the ground hard, leaving the horse to run two or three circuits of the track riderless.

But after being checked out by vets, Via Sistina was cleared to race, and went on to clinch one of the great Cox Plate wins.

The Ciaron Maher-trained Pride of Jenni had been expected to lead from the start, but instead Royal Patronage was the early pacesetter, before the former took the lead in the back straight.

Race favourite Prognosis raced up to be just behind Pride of Jenni and the pair looked set to battle it out, but McDonald made his move to make it a three-way go approaching the turn, before Pride of Jenni started to fade.

Prognosis was in front, but not for long as Via Sistina steamed on by to hit the front. The finishing straight at Moonee Valley is just 173m long, but in that short distance Via Sistina turned the race into a procession, with McDonald able to stand up in the irons and wave to the crowd well before the line.

"The truth is when she (Via Sistina) got up and James got up, the rest of it [I] didn't care [about]," he told Channel Seven.

"If she didn't make it [to the Cox Plate], who cares? She's alive and James is alive."

The victory gave McDonald his third straight win in the big race — following rides on Romantic Warrior and Anamoe in the last two years — and also marked the Australian jockey's 100th Group 1 triumph.

"I had such a smooth run the first half," McDonald told Channel Seven after the race.

"And I thought I'd just wheel it out, keep it smooth, blend in and she loved it. She won like Winx. She blended and then, yeah, just a phenomenal performance."

McDonald admitted he thought the trackwork incident was the end of things for Via Sistina as far as the Cox Plate went.

"I thought our hopes were doomed," he said.

"I think there was a fantastic picture there when I popped myself up and looked and watched her gallop away, and I think if looks could tell a story, we were stuffed."