Stewards have upheld a protest into alleged interference in the final 100 metres of the race that stops the nation.
Walker was also suspended for seven race meetings which will come into effect after Thursday's Kennedy Oaks race, Racing.com reported.
Prince Of Arran's trainer Charlie Felllowes has said in the past horses ridden by jockeys who break the whip rules should be disqualified, not just suspended.More news: Kris Jenner Denies O.J. Simpson Affair
The Australian four-year-old worked up the rails in the straight to edge out Prince of Arran and Il Paradiso after Master of Reality, who had finished second, was relegated to fourth following a stewards' protest.
Stewards have found that jockey Frankie Dettori blocked stablemate Il Paradiso in the run to the line, dumping the second place-finisher back to fourth.
NSW Greens Senator Ms Mehreen Faruqi took to Twitter to hit out at the excessive whipping.
"I don't know if Frankie is ever going to win this race".More news: RNC paid $60K for President Trump and guests to attend UFC 244
His victory aboard Vow And Declare in the Melbourne Cup saw him thwart the historic bid of jockey Damian Lane to win all four in the same calendar year.
In a desperate finish, Vow and Declare triumphed by half a head.
O'Brien said he had given Williams no specific instruction pre-race and that the jockey's decision to ride him up near the front of the pack had ultimately proved a masterstroke.More news: Fans loved the first episode of 'His Dark Materials' last night