Nonu was red carded by referee Steve Walsh in the 58th minute on Saturday for his tackle on Crusaders winger Tom Marshall.
"The tackle was executed poorly and there was no intention to hurt the opponent. Nonu accepted that the tackle was poorly executed by lifting his opponent," SANZAR judicial officer Jannie Lubbe said in his ruling.
"The tackled player was not injured and the incident had no effect on the Crusaders team. A medical report obtained from the Crusaders confirmed that the player involved suffered no injury.
"Taking into account the record of the player over a period of more than a decade playing Super Rugby, which is not unblemished, his early plea of guilt and the remorse shown, I have determined to give the player a discount of one week and to suspend the player from all rugby for a period of three weeks."
Highlanders flanker Jarrad Hoeata was sinbinned by referee Steve Walsh in the first half for illegal play while Crusaders hooker Corey Flynn was also sinbinned for a head-high tackle during the feisty game, which the Crusaders won 40-12.
The Highlanders are currently placed last and with two rounds of the regular season left, Nonu has played his last Super Rugby match this year.
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