Damien Oliver's tumultuous spring carnival took another twist on Melbourne Cup day when he was criticised for his ride on the favoured Americain while the threat of a long suspension hangs over him.

The Victorian government has been called on to step in to give stewards more authority to ban jockeys after reports Oliver had admitted betting on a rival horse were published on Cup day.

Oliver lost rides in the Cox Plate and Caulfield Cup when claims were first aired last month that he had placed a $10,000 bet on Miss Octopussy in a race at Moonee Valley two years ago, also prompting calls for him to be immediately banned.

But he found favour with some trainers, winning last week's Victoria Derby on Fiveandahalfstar and scoring the ride on 2010 Cup winner Americain on Tuesday.

But Americain's trainer Alain de Royer Dupre was disappointed with Oliver's ride in bringing home the French stayer, which started second favourite, in 11th place.

De Royer Dupre said Americain looked like he had not even had a run when he returned to the mounting yard.

Oliver had enjoyed success earlier in the day when he posted a win on Walk With Attitude in the Lexus Hybrid Plate, but he avoided media questions about the scandal.

"It's nice to get a win for (Americain's owner) Gerry (Ryan) before the Melbourne Cup," Oliver said as he returned to scale.

"I hope I can get a running double for him."

Oliver has known for some months he was under investigation, which became public when Fairfax Media reported he placed the bet.

On Tuesday, Fairfax reported he had admitted the bet to officials.

Ryan says he has not asked any questions of Oliver regarding the betting scandal.

Ryan called Oliver up to ride Americain in Tuesday's Cup after sacking Frenchman Gerald Mosse following the horse's fourth in the Caulfield Cup.

The Victorian opposition has called on the government to stop the scandal causing further damage to racing in the state.

Shadow racing minister Martin Pakula accused the government of "prevarication bordering on negligence" by refusing to grant racing authorities the powers to suspend jockeys.

Racing Victoria chief executive Rob Hines would not comment on the ongoing affair but said he expected the investigation to be completed soon.

And Victorian racing minister Denis Napthine said in a statement Racing Victoria operated separately from the government and it would be inappropriate for him to comment.

Mr Pakula said the government could introduce legislation which would give Racing Victoria the powers they have been asking for.

Mr Pakula said there was "absolutely no reason" to deny Racing Victoria the powers that Queensland Racing and New South Wales Racing already have.