A protest has denied Leon and Troy Corstens their first Group One win as a training partnership.

A week after stewards dismissed an Australian Guineas objection, Saturday's Group One result was overturned in favour of Preferment at the expense of the Corstens' outsider Awesome Rock.

Preferment's jockey Hugh Bowman claimed interference from the 400m against first past-the-post Awesome Rock, ridden by Stephen Baster.

A short half head separated the pair.

Chief steward Terry Bailey said Bowman was forced to ease off Awesome Rock's heels as it shifted out between the 300m and 200m, a move that affected the result.

Trainer Chris Waller said it not an ideal way to land his 12th Group One win for the season and he felt for the connections of Awesome Rock.

Preferment ($7) claimed his third Group One win at Flemington with his stewards' room win over Awesome Rock ($26). Rising Romance ($11) was 1-1/4 lengths away third.

"He didn't win the race rightfully but he was denied his fair chance in the race, but unfortunately there becomes a victim which was Awesome Rock," Waller said.

Waller said he was surprised Bowman took Preferment up to settle in second position behind Awesome Rock, but described that as the genius of the rider.

Preferment will now return to Sydney for the Queen Elizabeth Stakes after Waller revealed the Australian Cup victory came about because of a Racing Victoria incentive.

The four-year-old returned from a seventh placing in the Hong Kong Vase in December via the quarantine centre at Werribee racecourse where he was able to start work for an autumn campaign.

"We had the choice of taking him out of work to do his quarantine or remaining in work and doing his quarantine at Werribee," Waller said.

"They (Racing Victoria) covered half the quarantine costs if he ran in a Group One."

Bowman said Preferment was a genuine weight-for-age performer.

"He lacks a turn of foot but he makes up for that with his ability to gallop at his top for a long time." Bowman said.

Leon Corstens was disappointed that the result was overturned, denying Awesome Rock his first Group One win and a future as a stallion.

"It's on again next year and hopefully we'll get it back then," he said.

Damien Oliver, rider of the eighth-placed $3.10 favourite Bow Creek said the slow tempo was against his mount which made it hard to come from the back of the field after the import missed the start.