The retirement of Black Caviar leaves a gaping hole at the top of Australian racing which could deepen with the impending departure of those considered the next best.

Pierro goes into Saturday's Doncaster Mile as the favourite to maintain his unbeaten record in Sydney and the outcome of the race is likely to determine an announcement on his future stud career.

Stud duties beckon for fellow three-year-old All Too Hard, Black Caviar's younger half-brother who has been named on Vinery's 2013 stallion roster.

The colt inflicted the first defeat on Pierro in the Caulfield Guineas in the spring and like his rival, has won two Group One weight-for-age races this year.

Illness has so far robbed him of a third this autumn but he showed he had recovered with a barrier trial win at Randwick on Friday ahead of a scheduled start in the All Aged Stakes on Saturday week.

All Too Hard will meet More Joyous in the All Aged, a mare who, if not for Black Caviar, could reasonably have been rated the best since Makybe Diva.

The All Aged will determine whether All Too Hard goes to England for a tilt at the Queen Anne Stakes at Royal Ascot in June where a win would help secure a deal for northern hemisphere stud duties.

The outcome of the race could also mean More Joyous gets on the plane with him ahead of a date with Frankel who will serve a select group of mares to southern hemisphere time in September.

Pierro is also slated to run on Saturday week in the Queen Elizabeth Stakes which he would meet three-year-old triple crown winner It's A Dundeel.

Unlike Pierro and All Too Hard, the New Zealand colt has been kept to his own age group and the Queen Elizabeth will be his first test against older horses.

It's A Dundeel's dominant Australian Derby win propelled him to favouritism for the Cox Plate in the spring and the intention is to race him at four.

Although he has done all his racing bar one start in Australia, It's A Dundeel is rightly claimed by trainer Murray Baker as a New Zealander.

Australia might have to claim him as well.