It turned out to be the Group One win that almost wasn't for Sydney star Boban.

When it rained during the Caulfield meeting which ushered in the first Group One race of the new Australian racing season on Saturday, Chris Waller thought Boban's chances of winning at the elite level for the fifth time had been washed away.

Waller, who won 14 Group One races in 2014/2015, was at Rosehill races and when he was told Caulfield had been downgraded before the Memsie Stakes he was on the telephone to Melbourne trying to get an accurate reading on just how much the surface had deteriorated.

He took a punt, much in the same way his jockey Glyn Schofield did when he steered Boban along the fence to win the $400,000 weight-for-age race in a three-way finish.

"In the end there's nothing for him in the next couple of weeks so we decided to roll the dice," Waller said.

"And it came down to a great ride from Glyn."

Boban has never been at his best on a rain-affected track but he was still backed into favouritism at $5.50 before beating Entirely Platinum ($9.50) by a short head with Stratum Star ($8) a nose away third.

The Memsie was Boban's first start since a Queensland winter campaign when he won the Doomben 10,000 and finished second in the Stradbroke Handicap.

"He is a bloody fantastic horse," Waller said. "Not many can win five Group Ones."

Schofield had Boban buried away in a midfield position on the fence and he gambled and won on the inside not being too chopped up because he reasoned it was going to be hard to make a run outside the leaders when horses still in contention were holding their ground.

"We had a chat beforehand and there had been a couple of horses finish off quite well on the inside and whilst they hadn't won they had filled some minor placings," Schofield said.

"I thought he wasn't going to win going around them, so I'm quite fortunate I stayed where I was and we got there in time."

Schofield said Boban was a rejuvenated racehorse since it was decided to keep him to sprint races after a bleeding attack in the 2014 Queen Elizabeth Stakes.

"He got a lot of confidence through Brisbane," Schofield said.

Dwayne Dunn said Entirely Platinum, chasing his maiden Group One victory, was gallant in defeat.

"I thought he was home today," Dunn said.

"To his credit he was softened up early by a horse that normally races back."