Comeback galloper Rain Drum has foiled the hopes of several horses hoping to guarantee themselves an Epsom Handicap start with an upset victory in Saturday's Group Two Shannon Stakes at Rosehill.

In a competitive finish, the Gai Waterhouse-trained Rain Drum ($20) dived through nearest the inside to get the verdict by a head over veteran Centennial Park ($19) whose second placing tipped him over the $1 million prize money mark.

Our Desert Warrior ($9) was another long head back in third.

The winner is exempt from an Epsom ballot but it will be of no use to Rain Drum who is not nominated.

Our Desert Warrior, Hosting and Your Honour were all racing to secure the ballot exemption and their connections now face an anxious wait to see if they can secure a start in next Saturday's Group One.

The Shannon Stakes (1500m) was Rain Drum's first race in 16 months.

The six-year-old injured a suspensory after he won at Rosehill in May 2012 and was tackling the 1500m race first-up, prompting winning jockey Peter Robl to praise the training prowess of Waterhouse.

"All credit goes to Gai. It's an amazing training performance and that's why she's the first lady of racing," Robl said.

"He was strong to the line and he travelled beautifully in the run.

"I was held up until the 250 (metres) which has probably helped him in the end."

Waterhouse hasn't set any concrete plans for Rain Drum and will see how he comes through the race before deciding on his next target.

She is hoping Our Desert Warrior, 86th in the order of entry going into the Shannon, can sneak into the Epsom field.

His jockey Tommy Berry believes he will be competitive in the Group One.

"If you got in, you'd run him," Berry said to Waterhouse.

Victorian Your Honour started the $3 favourite and finished sixth, one place behind his Greg Eurell-trained stablemate Hosting who had to switch around heels early in the straight and was solid to the line.

The disappointment of the race was High Esteem who had been prominent in Epsom markets but finished second last after racing behind the leader.