Champion jockey Glen Boss will need to call on all the tactical nous and skill that has won him five Doncasters if he's to add to his record haul in Australian racing's feature mile handicap race on Saturday.

Boss knows as well as anyone what it takes to win the Doncaster at Randwick and was counting on a low barrier draw to boost his chances of adding to his record, aboard Streama for astute trainer Guy Walter.

He didn't get it.

Streama has barrier 18.

Boss passed up the chance of riding Group One mile handicap winner Happy Trails to link with Streama.

The jockey's faith in Walter's ability to have his horses at their peak for the big occasion weighed heavily.

"That swayed me a lot," Boss said.

"Obviously I've been talking to Guy for quite a while because I knew she would be getting in with a light weight.

"And that's what swayed me because this has always been her target. Not that this hasn't been the target for a lot of horses, but Guy is very, very good at it."

Walter also has his own place in Doncaster history having trained the trifecta in 2005.

Streama is yet to win a race this campaign but Boss believes she has "a good profile" for the race although she'll need plenty of luck for the wide gate.

The four-year-old has 53.5kg having carried 57kg or more at her past four starts.

"Her best win was the Flight Stakes over a mile there," Boss said.

"And Guy Walter is in flying form. His horses are all going great."