The country's leading Group One rider Nash Rawiller will be the new partner for Southern Speed when she defends her Caulfield Cup title on Saturday week.

Rawiller replaces Glen Boss who jumped off the mare to chase the ride on Metropolitan winner Glencadam Gold in both the Caulfield and Melbourne Cups.

He was unsuccessful with Jim Cassidy getting the nod on the Cups favourite.

Trainer Leon Macdonald was mystified by Southern Speed's lacklustre performance in Saturday's Turnbull Stakes when she beat just one runner home but said he was sure the mare could turn it around.

"I'm more than happy to have a rider of Nash's calibre aboard," Macdonald said.

"He's won a Caulfield Cup before and is a great jockey."

Rawiller won the 2004 Caulfield Cup aboard Elvstroem, the horse which thrust him into the limelight and helped his rise to the top of Australia's riding ranks.

He claimed the title of leading Group One jockey for season 2011/12 over Black Caviar's rider Luke Nolen.

Macdonald said no problems had surfaced with Southern Speed three days after what he described as a career low.

"The vets have been over her and the chiropractor has been over her with a fine tooth comb and there is nothing wrong," he said.

"We've given her a couple of easy days on the water walker and she is fine."

Southern Speed is now a $17 chance for the Caulfield Cup with Glencadam Gold favourite at $4.80 following Tuesday's third declarations.

Stable jockey Tommy Berry rode the Gai Waterhouse-trained Glencadam Gold to victory in the Metropolitan.

The win lifted Glencadam Gold's weight to 53.5kg for the Cups, outside the weight range of Rawiller who is Waterhouse's No.1 rider.