Irish trainer Aidan O'Brien's travelling foreman TJ Comerford says only a bad day can stop Winx from winning a historic fourth Cox Plate at Moonee Valley.

O'Brien and Comerford know what's required to win Australia's premier weight-for-age race having won with Adelaide in 2014 and on Saturday tackle the great mare with Rostropovich.

"I don't think she is vulnerable, bar she has a bad day herself," Comerford said.

"And she hasn't had one of those in 28 starts, so I doubt she's going to have one here."

Comerford knows from experience the difficulties in tackling Winx, finishing third to the mare with Highland Reel in the first of her Cox Plate's in 2015.

Highland Reel won greater prizes with Comerford saying he was a good horse.

"But Winx is very good," he said.

"It's great to run against her. She's a great filly and we're privileged to take part in the race more than anything."

Arriving on the last shipment to Australia for the Melbourne spring, Rostropovich finishes quarantine on Saturday.

Comerford said Rostropovich's form compared favourably with that of Adelaide before his 2014 success.

But Comerford said the Melbourne Cup on November 6 was Rostropovich's target.

"He's in good form, in good shape, he looks well," Comerford said.

"We're going to let him take his chance and see what he can do to keep him right for the Melbourne Cup.

"I thought if he could run into a place, that would be a big run especially heading into a Melbourne Cup.

"I wouldn't be disappointed if he beat Winx but I probably would be a bit surprised."

Comerford said Godolphin galloper Benbatl was a worthy contender adding the Caulfield Stakes winner was her main danger.

Winx is the $1.20 favourite in latest betting with Benbatl at $8, the only other contender under double figure odds.

"He's a Group One winner and you have to respect him," Comerford said.

"We're probably getting a bit of weight of them both so that gives us a bit of a chance.

"But I'd say she (Winx) just has to go down to the start, turn around and come back."

Rostropovich is rated at $31, a line of betting he shares with Dargento and Savvy Coup.