Fears about track conditions being too hard for Big Bonanza proved unfounded when the gelding charged down the the straight to win the final race at Rosehill.

Big Bonanza made a late charge to push past front-runner Pentasia and secure the Harvey Norman Sprint (1200m) in an impressive victory.

But the winning horse almost didn't make it to the race. There were thoughts of scratching Big Bonanza when the track was upgraded to a good (2).

Trainer Don Robb thought better of making a formal application to stewards, and the decision paid off.

"He's a very genuine horse, and the owners have been stickers," Robb said. "We're really over the moon."

Fate was certainly with the horse, with injured jockey Nigel Seymour overcoming a sore ankle and "a couple of bruises" from a fall at Scone on Tuesday.

"I couldn't walk through the day yesterday and I let the physio back on to my leg and it's just sort of come good," he told AAP.

Seymour said while he was sceptical about whether he could perform on the day, a well planned attack and a drop from 57kg to 54.5kg for Big Bonanza from his last start combined to bring out the winning result.

"Everything worked out to plan," Seymour said. "We were hoping to get up to third or fourth, run off the fence and get a kick when it did, and hopefully we could run it down."

"He's been a very good horse for me, quite a good horse."

Big Bonanza's win came minutes after fellow Wyong trained horse Sincero failed to run a place in the Cox Plate.

"Poor old Mick (jockey Michael Rodd) has got a long face over there, but some of us win and some of us lose," Robb said.

"But you know, we're going all right."