Colourful owner John Singleton has launched a massive Melbourne Cup plunge, backing his mare Dear Demi to win more than $3 million.

In one bet negotiated directly with Sportingbet's Australian manager Michael Sullivan, Singleton had $50,000 on Dear Demi at $41. He also had $25,000 for a place on the mare.

Singleton also backed Dear Demi to win another million with TopSport.

Sullivan said Singleton described his punt as a "sentimental" wager on a filly who has become one of his favourites.

Bookies have reacted to the Singleton money by turning Dear Demi in to $26 for Tuesday week's race.

Dear Demi underlined her Melbourne Cup credentials with a strong-finishing third to Fawkner in the Caulfield Cup last Saturday.

Last year's VRC Oaks winner started from the widest barrier in the Caulfield Cup and made up an impressive amount of ground along the inside.

Singleton's betting spree came as it was confirmed Brisbane jockey Chris Munce would ride Dear Demi in the Melbourne Cup in which she has the featherweight of 51kg.

Munce will ride Dear Demi for the first time when she runs in the Mackinnon Stakes at Flemington on Saturday week.

He will be chasing his second Melbourne Cup win after partnering Jezabeel to victory in 1998.