In a result that is sure to haunt Irish training maestro Aidan O'Brien, Sir Dragonet beat former barnmate Armory in Australia's most prestigious race.

Sold to a high-profile syndicate attached to the emerging powerhouse stable of Ciaron Maher and David Eustace, Sir Dragonet arrived in Australia with a reputation as costly conveyance on the racetrack.

He was rated good enough to start favourite in last year's English Derby but finished out of the placings.

But all the time he remained on the radar of Melbourne property developer Ozzie Kheir.

Kheir finally struck a deal with global thoroughbred operation Coolmore, paving the way for Sir Dragonet to make one of the more stunning racing debuts on Australian soil.

In a race decided on testing ground because of rain, Sir Dragonet ($7.50) burst onto the scene between horses and in a cruel twist for his former yard, gathered in the O'Brien-trained Armory ($7.50) to win by 1-1/4 lengths.

The favourite Russian Camelot ($3.50) ran bravely to take the minor placing to complete a trifecta for northern hemisphere-bred horses.

It was a fourth Cox Plate triumph for veteran jockey Glen Boss after winning on Makybe Diva (2005), So You Think (2009) and Ocean Park in 2012.

For Boss, it will go down as his most unexpected after answering an SOS from Sir Dragonet's owners when Hugh Bowman was suspended.

"They put up a few names and Hughie backed me. He said 'put Bossy on'. And thank God I got the job," he said.

Boss now joins Bowman among a quartet as the Cox Plate's second most successful jockeys behind Darby Munro.

He gave Sir Dragonet a perfect ride, saving ground at every opportunity before latching onto the back of Armory when the sprint went on.

Armory was being hailed the winner at the 200m until Boss drove Sir Dragonet between runners and a surge at the 100m put the race beyond doubt.

"I couldn't have scripted the run any better. He just gave me a lovely ride ... I was just twitching my fingers throughout the race," he said.

A three-time Melbourne Cup-winning jockey thanks to the legendary exploits of Makybe Diva, Boss reckons Sir Dragonet can do it for him again at Flemington.

"He has great form, and he might go a little better in the Melbourne Cup, because he was just starting to get warmed up towards the line.

"The way he gave me a feel today, he might be running in the Melbourne Cup and he might be running very well."

Boss' win didn't come without some cost.

He was fined $1000 for his celebration as Sir Dragonet crossed the line in front of grandstands left empty because of strict Victorian lockdown measures.

A breach of COVID-19 protocols also left the jockey lighter in the pocket with another $1000 penalty.

For Maher and Eustace, winning a Cox Plate only confirms the meteoric rise their stable has enjoyed over the past two seasons.

"To think you get a horse of this calibre. I've looked after him for a bit over two weeks - it is just phenomenal," Maher said.

Damien Oliver had nothing but the highest praise for Russian Camelot.

"He was super brave even when the first two came to him. He fought it out really well," the champion jockey said.

Australian Associated Press