Corey Brown is keen to avoid past crash dieting stress as he begins preparations to shed three kilograms to ride race favourite Pierro in the Cox Plate.

The champion jockey will begin a strict diet and exercise regime to reach 49.5kg to ride the three-year-old colt, once he wraps up the Caulfield Cup meeting on Saturday.

Brown was selected to ride the Cox Plate favourite after Craig Williams, who cited the weight restriction as an issue, pulled out to ride second favourite Green Moon.

Brown agreed to the partnership after receiving a late night call from Pierro's trainer Gai Waterhouse on Monday.

He's now set to shed about 3kg in less than six days through a mixture of diet and exercise, with plenty of steamed vegetables, steamed fish and soup.

"I want to do it step by step," Brown told AAP.

"The food I wouldn't say cuts down by half, maybe only a quarter, but it becomes lot more healthier so it goes through you quicker.

"Then it's just a lot of training, a lot of walking, running, and I'll throw in a bit of swimming."

By Tuesday Brown says he'll be down to 51kg, by Thursday around 50, and by showtime on Saturday, the correct weight.

Brown says he's done it before and can do it again.

"Like I said, it's not like I've grown any, it's hanging there to get off," he said.

But he's also conscious of avoiding past crash dieting stress, like his preparation for the 1998 Melbourne Cup when he dropped from 50.5kg to 47.5kg in three days to ride Victoria Derby winner Arena.

"When I rode 47-1/2 a few years ago in Melbourne, it was the wrong way to do it," he said.

"Basically I crash dieted. I rode in the race but basically on pilot."

He's also to keen to avoid too many races on the day before the big one.

"I'll only have three or four max, because by the time I get to the grand final, I don't want to be toasted."

He'll grant himself one last indulgence before he starts the regime.

"I'm going to try and have a good meal on Saturday night," Brown said.

"Obviously a piece of steak - I love meat so I'll be having a decent meal after the Caulfield Cup."

But if the regime proves tough, there's plenty of motivation in the rewards at the end of the race, he says.

"Like you see, there's a big carrot dangling at the end of the rainbow, so to speak."