Distressing vision of one of Darren Weir's prized horses being tormented with an electric prod in preparation for last year's Melbourne Cup may be a crucial piece of evidence in the case against the disgraced trainer and two of his associates.

The emergence of the video at the start of a new spring carnival directly links the Weir scandal to Australia's greatest horse race.

Red Cardinal's trainer, Darren Weir (right), in the 2018 Melbourne Cup parade. Credit:Getty Images

Video footage obtained by Victoria Police detectives and shown to racing authorities shows Red Cardinal being "jiggered" in the neck by stablehand Tyson Kermond as it "jog trots" on a treadmill inside Weir's Warrnambool stables, sources have told The Age and The Sydney Morning Herald.

The footage, which runs for about 20 seconds, allegedly shows Weir and stable foreman Jarrod McLean looking on as the horse is repeatedly subjected to the painful ordeal on the weekend before the running of the Cup.