A magic milestone for the jockey, justification for his new owners and the right to be known as Australia's No.1 sprinter.

In almost every way Zoustar achieved more by winning the Coolmore Stud Stakes (1200m) than any other horse this spring - including next Tuesday's Melbourne Cup champion.

Zoustar's victory came at the end of a week in which he was sold for $14 million to Widden Stud.

But that is only part of the deal.

Under the terms of the sale to Widden Stud, his win in Saturday's Group One at Flemington earned his owners an additional $4 million, and there's another $2 million on the table if he can add another Group One race to his record.

While the head-spinning numbers caused one of his owners to faint and made Zoustar one of Australia's most valuable thoroughbreds, the human side of his win is equally astonishing.

The win is jockey Jim Cassidy's 100th at racing's top level, a feat achieved previously by only by two of the all-time greats of the Australian turf, Roy Higgins and George Moore.

Trainer Chris Waller revealed that during the sale process Zoustar had endured various examinations, "including to his reproductive organs".

Cassidy sympathised after the win suggesting both he and the horse had their vital parts on the line.

"I've never been so relieved to win a Group One race in my life," Cassidy said.

Zoustar will now be rested with a view to following Australia's previous best galloper, Black Caviar, to Royal Ascot.

Waller said plans hadn't yet been finalised, but Ascot would be his recommendation.

"Fortunately we've got a realistic chance, so that's the dream," Waller said.

Along with the $300,000 he earned directly for winning Saturday's race, Zoustar's victory earned his old owners $4.3 million, well above the $3.6 million that will go to the Melbourne Cup winner.

Zoustar, the $2 favourite, raced down the middle of the Flemington straight course, cruising to the lead at the 300m and stretching his advantage to two lengths over Not Listenin'tome on the line.

Lion Of Belfort ($15) ran on from last to finish third, a further head away.

As impressive as Zoustar may have been, Waller predicted even more in the autumn.

"He's still not quite there yet ... still learning," Waller said.

"He didn't get any cover which didn't help, he was always going to be vulnerable the last 200m.

"I think there's still a lot of improvement to come."

Waller gave more praise than he accepted for Zoustar's win, paying special attention to Cassidy.

"It's just a huge privilege to be there and it's good for such a great horse to crack it for Jimmy," he said.

"Obviously every horse is a good horse to win a Group One race, and he has won 100 of them. That's just unbelievable."