The ARU will appeal Wallabies lock Rob Simmons's eight-week suspension for a lifting tackle on French forward Yannick Nyanga in Paris two weeks ago.
The appeal, lodged from Florence on Wednesday, will be heard by an IRB-appointed three-man committee, a Six Nations statement said.
Simmons stayed with the squad while the Wallabies waited for the detailed judgement to arrive from the Six Nations committee that oversaw the case.
The Wallabies, who are also without injured lock Kane Douglas, last week called up Rebels lock Cadeyrn Neville to provide back up in the second row.
Scott Higginbotham, a back rower, also joined the team at the weekend after serving a two-Test ban for kneeing and head-butting All Blacks captain Richie McCaw.
The decision to appeal the Simmons suspension came as the IRB announced it would contest what it claimed is an "unduly lenient" one-week ban on All Blacks forward Adam Thomson.
The New Zealand blindside flanker was yellow-carded for trampling the head of Scotland opponent Alasdair Strokosh during the All Blacks Test against at Murrayfield 11 days ago.
"After careful consideration and having reviewed the full written decision in the Thomson case well within the permitted 72 hours of receipt, the IRB strongly believes that the sanction of one week is unduly lenient for this particular act of foul play and not aligned with the sanctions handed down in similar cases," a statement from the board said.