Unraced outsider Empress O'Reilly had to survive a protest and a late rider replacement to win the Joe Brown Plate for two-year-olds at Caulfield on Thursday.

The filly's declared jockey, apprentice Thomas Sadler, couldn't make the 55kg weight and it fell to fellow junior rider Katelyn Mallyon to climb aboard just a few minutes before the race.

But the drama didn't stop there for Empress O'Reilly.

Starting at $26, the filly led from the start of the 1000-metre race but shied abruptly close to the line and shifted out, making contact with the fast-finishing Fontein Ruby ($5.50) whose jockey Ryan Maloney fired in a protest against the winner.

Fontein Ruby's trainer Tony Vasil told stewards the incident caused his runner-up to lose momentum and probably cost her half a length.

However, stewards wasted little time in dismissing the objection saying although Empress O'Reilly did shift, the interference came too close to the finishing line to change the result.

Trainer Robert Smerdon admitted he was surprised by Empress O'Reilly's brave performance saying she had looked a little fat to his eye, causing him to ponder whether she was fit enough to race.

"She showed us good ability at home in her jump-outs and whilst she was capable of being in the firing line for a fair way, I felt her lack of condition may have gone against her," he said.

But Empress O'Reilly, who cost $32,000 at the New Zealand select sale, proved him wrong as she hung on to win by a long neck from Fontein Ruby with La Cicciolina ($14) third another 1-3/4 lengths away.

Smerdon had favoured the chances of his other runner in the race, Cool `n' Sassy ($6) who finished eighth.

"I thought both of them would benefit from the experience. Both have shown promise and prospects, I wanted to see how they lined up," Smerdon said.