(CNN) -- Melbourne Cup-winning jockey Michelle Payne has been banned from competing for four weeks following a positive drugs test.

The Australian was handed the suspension by Racing Victoria after she tested positive for the banned appetite suppressant Phentermine.

Payne, who in 2015 became the first female to win the Melbourne Cup, pleaded guilty at Thursday's inquiry held at Flemington racecourse, the site of her famous win on 100-1 shot Prince of Penzance.

"I would like to take full responsibility and had I known it would still be my system, and I would be riding with a banned substance in my system, I wouldn't be riding," Payne said.

"I'm embarrassed and I'm sorry for that.

"The onus is 100% with me as a rider to know what I am taking and the rules around it regardless of whether it has been prescribed to me or not."

The positive sample was provided by the 31-year-old at the Swan Hill Cup meeting in Victoria on June 11, nine days before she guided Kaspersky to fifth place in the Queen Anne Stakes at Royal Ascot.

Payne is banned until July 21 and is scheduled to return to racing at Ascot for the Shergar Cup on August 12.

Just last week, the Australian told CNN she was now aiming to leave the saddle and win the Melbourne Cup as a trainer.

"The training is something I've been planning towards a long time now," she said. "I'm preparing for the riding to come to an end.

"It will be hard to say goodbye but I've had a number of bad injuries and obviously one day I want to have a family of my own. But it's a long-range plan so I'm not stopping just yet."