Lloyd Williams must be wishing Prince Charles comes to every Melbourne Cup.

The former casino owner and prolific horse owner won his fourth Cup when Green Moon gave jockey Brett Prebble the ride of his life at Flemington on Tuesday to upstage the international-laden field and keep the trophy in Australia.

He upstaged most punters too, winning at $22.50 from Irish stayers Fiorente and Jakkalberry while the French pair of pre-race favourites, Dunaden and Americain, finished well back.

The last time Charles was at Flemington, he presented Williams with his second Melbourne Cup when What A Nuisance won in 1985.

This time he let wife Camilla, Duchess of Cornwall, perform the honours while Williams' son Nick stepped up for his father who watched the race from his rambling training property at Mt Macedon to Melbourne's north.

Nick also collected the Cup in 2007 when Efficient won, while Lloyd was on hand for his first Melbourne Cup win with Just A Dash in 1981.

The forecast storm held off in time for Camilla to present the Cup after the crowd of about 100,000 had looked anxiously above when a clap of thunder rang out as the trainers addressed the jockeys in the mounting yard.

But Green Moon shone under the grey skies as Prebble took the lead at the 200m mark to win by a length from Fiorente which gave trainer Gai Waterhouse her third Melbourne Cup runner-up.

As Hollywood stars like Mischa Barton and Rose Byrne joined a host of Australian celebrities watching on from the Birdcage marquees, European stayer Jakkalberry finished third, just over two lengths from the winner.

Bred in Ireland, Green Moon can be considered an honorary Australian, a resident of Mt Macedon since Williams bought him 18 months ago with a plan to win this Melbourne Cup.

"He's a great strategist in everything he does," Nick said of his father.

"It's another Melbourne Cup triumph for Team Williams and I'm just ecstatic.

"We put a lot of money in and a heck of a lot of effort so to get this result today is terrific."

Williams the junior pointed out four Melbourne Cups is not as simple as it might sound.

"He's (Lloyd) had close to 75-80 runners in the Melbourne Cup, he's won four. It's not easy."

Hong Kong based Victorian Prebble described his passage on Green Moon as "the ride of my life" and the result "my life dream".

He also paid tribute to Lloyd Williams.

"That's for you boss. Thank you for your support, I can't thank you enough," Prebble said.

"It's the pinnacle of racing in Australia. I've had some great highs here and you never think you can get any higher until you get to the pinnacle."

Meanwhile, the boss sat back at Mt Macedon, pondering another $3.6 million winner's cheque. He might put some of it toward an airfare for the prince next year.