Whether it is a case of mind games or genuine doubt, the normally ebullient Gai Waterhouse has flagged concerns over the weight Pierro has been asked to carry in Saturday's Doncaster Mile.

The colt has 57kg, 1kg less than topweight Shoot Out and 4kg over the limit.

More tellingly, the last two three-year-olds to have won the famous 1600m feature, Triple Honour (2008) and Haradasun (2007), carried 51kg and 53kg respectively.

Neither had the credentials of Pierro who has won five Group One races.

But Waterhouse still believes her star colt has been presented with a significant challenge at Randwick on Saturday.

"Weight stops a train, will it stop Pierro?" Waterhouse said.

"Fifty seven kilograms. I can't think of a three-year-old in recent history who has been asked to lumber such a big weight. It's a massive weight but he's earned it."

Waterhouse will be chasing her eighth Doncaster Mile victory and second in succession following More Joyous' triumph 12 months ago.

Pharaoh in 1994 gave the Randwick trainer her first win in the race and Waterhouse says the Doncaster has changed markedly since then.

"It's not a true handicap like it used to be. It's a fool's handicap in a way," she said.

"I think it should be a full weight scale like in England."

Waterhouse estimated More Joyous would have been given "62 or 63 kilos" if connections had elected to chase back-to-back Doncaster wins and said handicappers shouldn't be afraid to give class horses big weights.

"But they won't do it because trainers blow up and they shouldn't," she said.

"Horses deserve it and it makes it more interesting.

"It probably just means the good horses don't run in handicaps, they run in the weight-for-age races."

A capacity field of 20 plus one emergency will tackle Saturday's Doncaster, one of two Group One races on the program along with the Australian Oaks.

While Waterhouse has expressed concerns over the weight given to Pierro, the TAB has him as the firm $2.10 favourite ahead of filly Norzita at $8.