The ban will be held over until the start of next year as Australia are not scheduled to play again until then.
Kepu was shown a red card after his badly mistimed challenge ended with Watson being struck in the side of the head.
Scotland went on to record their biggest victory against the Wallabies in 90 years, with Australia coach Michael Cheika afterwards admitting ‘the referee really didn’t have much alternative’ but to dismiss Kepu for the over-the-top-tackle.
The 31-year-old appeared before an independent World Rugby-appointed disciplinary committee on Tuesday, to answer against an infringement of law 10.4 (h), which states ‘a player must not charge into a ruck or maul. Charging includes any contact made without use of the arms, or without grasping a player’.
Kepu, the committee said, had accepted the tackle - considered to have been ‘a dangerous shoulder charge into a ruck in which contact was made with an opponent’s head’ - was an act of foul play which had warranted a red card.
Following Kepu’s contrition and previously clean disciplinary record, the disciplinary committee decided to allow the ‘maximum discount of 50 per cent and reduced the length of the suspension to three weeks’.
However, with the Southern Hemisphere about to enter its close season, the disciplinary committee ruled to suspend Kepu until midnight on March 4 2018. The player has the right of appeal.