"It was a very good run and he carried 2kg more than anything that finished in front of him," O'Sullivan said.
The Savabeel gelding had previously run third in the group 1 New Zealand Stakes behind Lizzie L'Amour and Benzini.
"Through nobody's fault, he was going backwards between the 800 and 600m when he should have been going forwards so he could have made it interesting with a bit more luck," O'Sullivan said.
"He's maturing now, he's always been a very weak racehorse."
Testament to Sound Proposition's talent was his third placing in the group 1 New Zealand Derby behind Mongolian Khan and Volkstok'n'barrell.
Meanwhile, two of the stable's other leading lights this season are enjoying time out.
Sistema Stakes winner Summer Passage came home to the paddock after his runner-up finish in the group 1 Sires' Produce Stakes at Randwick and Charles Road has also been turned out.
"He's spelling, but he won't be out for too long and we'll have him back to get ready for Melbourne in the spring," O'Sullivan said.
Charles Road won the group 3 Manawatu Classic last month before he finished second in the Championship Stakes at Ellerslie.
Meanwhile, 3-year-old Tavidream, who is under offer, is another on his way to Brisbane, regardless of a change of ownership.
An Australian interest has an option to purchase the son of Tavistock, who has won five of his seven starts.
"There's a contract in place on him and part of the deal is that we take him to Brisbane for the Derby," said Donna Logan, who trains Tavidream with Chris Gibbs.
"He'll be on the plane on Thursday. He's very bright and he's done extremely well since his last run."
At his most recent appearance, Tavidream beat the group 3 Manawatu Classic winner Charles Road in the group 2 Championship Stakes at Ellerslie.
"He'll just have the one lead-up run to the Derby in the Grand Prix Stakes at Eagle Farm on May 27," Logan said.
Stablemate and three-time winner Comin'through also has the group 1 Queensland Derby on his programme and he will open his campaign in the group 3 Rough Habit Plate at Doomben on May 13.
Logan also has a classic plan for Jake The Muss, who has won only one race and was a last-start fourth in the Championship Stakes.
"He's probably going to go to Sydney first and try and qualify there for the Queensland Derby," she said.
The other member of the travelling party for Queensland is Qiji Phoenix, whose target is the group 1 J.J. Atkins at Eagle Farm on June 10.
"He's doing very well, he looks pretty special," Logan said.
An unbeaten colt by Not A Single Doubt, Qiji Phoenix completed a hat-trick of wins with a clear-cut victory in the listed Champagne Stakes at Ellerslie.
- NZ Racing Desk
