Australian lock Rob Simmons had his ban for a dangerous tip-tackle during the Wallabies defeat by France this month cut from eight weeks to five weeks.
His reduced suspension means Simmons will still be banned from the concluding match of the Wallabies' European tour, against Wales in Cardiff on Saturday, with the second row having already missed the wins over England and Italy.
On as a replacement, Simmons was cited by South African match official Freek Burger for his challenge on Yannick Nyanga midway through the second half of Australia's tour-opening 33-6 loss to France in Paris on November 10.
During the course of the match, Welsh referee Nigel Owens was heard apologising for the failure to issue a red card for the challenge on Nyanga due to a collective inability to identify tackler Simmons.
The original disciplinary hearing determined 23-year-old Simmons had been guilty of an offence at the "high end entry point" of seriousness.
An appeal hearing chaired by lawyer Chris Quinlan of England, which started on Tuesday but did not publish its verdict until the early hours of Wednesday morning, found Simmons, a veteran of 23 Tests, had committed the act of foul play and so dismissed the player's appeal on that point.
But a statement issued by the Six Nations Committee, which oversees disciplinary matters on behalf of the IRB during the European year-end internationals, added the appeal committee "had found that the player's offending had merited a mid-range entry point, rather than the top end entry point that the judicial officer had originally found".
It added: "After applying aggravating and mitigating factors, the appeal committee imposed a reduced sanction of five weeks.
"Therefore the player will be free to play from February 4, 2013."
As was the case with the original ban, the reduced punishment takes account of the close season inactivity after the end of the Wallaby tour when Williams and his Wallaby team-mates are not scheduled to play.
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