Chief steward Ray Murrihy has put James McDonald on notice over the jockey's selectiveness when it comes to accepting rides down in the weights.

McDonald left David Pfieffer searching for a replacement rider on Atmospherical in the Sapphire Stakes at Randwick on Saturday when he told the young trainer he couldn't make the mare's 54kg in the $300,000 race.

The Godolphin rider took three mounts at Atmospherical's weight on Monday but said he was never going to ride at 54kg on the second day of The Championships.

"I know you have difficulty (with your weight), I'm not saying you don't," Murrihy said.

"But we can't have a situation where you are picking and choosing who you ride for at 54 and you accepting a ride at 54 and you letting someone down."

The No.1 stable rider for Godolphin trainer John O'Shea, McDonald insists he told his manager he wouldn't be able to accept lightweight rides on Saturday despite being on three mounts, including top-class filly First Seal, which have 54.5kg.

"I said to David Raphael I didn't want to ride light. I don't ride at my best at 54 (kilograms) and I won't be able to make 54 on Saturday," McDonald said.

Murrihy interjected: "Obviously if Mr O'Shea wants you to ride at 54 you do.

"I suggest what you've got to do is go to your manager, put it in writing or whatever, saying what days you are going to be able to ride 54 and what days you aren't."

Pfieffer has engaged Doncaster Mile-winning jockey Glen Boss to replace McDonald but Murrihy says the trainer should not have been in the position of having to find a rider after acceptance time on Wednesday.

"There'll be other riders but he wants you. He booked you a fortnight ago," Murrihy told McDonald before issuing a $400 fine.

McDonald leads the race for the Nathan Berry Medal, awarded to the leading jockey over the two days of The Championships.

His Group One rides on Saturday include Queen Elizabeth Stakes favourite Contributer, New Zealand filly Savaria in the ATC Australian Oaks and First Seal in the Queen Of The Turf Stakes.