Seven years after he won his first Doncaster Mile with Triple Honour, trainer Chris Waller has notched No.5 with a horse not guaranteed a start until the last minute.

The second emergency in the $3 million race, Kermadec was safely in the field on Saturday but when the Randwick meeting was transferred to Monday, scratchings were reinstated.

To punters' relief, the same horses were withdrawn and three-year-old Kermadec went to the gates as one of the leading chances at $8.

Getting him in the field was outside Waller's control but getting him to win was an achievement that made Sydney's premier trainer justifiably proud, and a little emotional.

Kermadec was ridden by Glen Boss who steered three-year-old Triple Honour to win the 2008 Doncaster, a race which set Waller on his way.

"He is an exciting horse. Whether he got a start or not was out of my control," Waller said.

"I know he is a Group One horse and if he didn't get his chance today, it would have come another day.

"Group One horses find their races.

"But it is wonderful to see these colours win. His owners have been with me a long time.

"And Glen won my first Group One in this race. It's very special."

It was the second Doncaster win for Kermadec's owner Neville Morgan who raced Rangirangdoo, the 2010 winner, and the sixth for Boss who got Kermadec into a comfortable position from barrier 18 of 20.

As the field made the run for home, Boss produced Kermadec to make his run down the middle of the heavy track and the colt responded, charging to a 1-3/4 length win over Japanese visitor Real Impact ($10).

The Waller-trained Royal Descent ($8), second in the race last year, ran a gallant third with Queenslander Rudy ($19) fourth.

Pornichet, the $7 favourite, finished fifth while Hallowed Crown ($8) was 10th after getting a long way back early.

By his own standards, three-time Melbourne Cup winner Boss has had a lean couple of years but the momentum has picked up with his recent win in the William Reid Stakes aboard Lucky Hussler.

Boss said his ability to ride at a light weight was a factor in his Doncaster success over the years along with the ability to find the right horse.

"I was prepared to roll the dice. I really wanted to ride this horse," Boss said.

"I've had a lack of opportunities but it's one of those things and you work through it.

"It's nice to be a part of this horse and to win this for Chris.

"I said back when I won on Triple Honour Chris would be a superstar.

"He is something special like Gai. They are prepared to go that extra bit to be the best."

Boss is prepared to do the same but came back to scale 0.7kg over the weight he went out on, causing a $1000 blip on his perfect day when he was fined by stewards.