The United midfielder - currently on loan at Birmingham City - watched on as Mitchell Duke's superb first-half header proved the difference between the two sides, despite Tunisia's best efforts to get level.
The Socceroos, marshalled by the resilient centre-back partnership of Harry Souttar and Kye Rowles, and goalkeeper Matt Ryan, saw out the vital victory by thwarting attempts from Mohamed Drager, Youssef Msakni, Naim Sliti and Wahbi Khazri, as the Socceroos moved to second in Group D.
The Eagles of Carthage threw a lot of players forward in the closing stages in the search for a leveller, but came up short, meaning that nothing less than victory against holders France on Wednesday will be enough to progress to the Round of 16.