The home side ran in seven tries for their biggest ever victory over the Wallabies, who have been depleted by injury and player absences for personal reasons.
Rennie will have learnt a lot from seeing his next best in action, but with the future of regular skipper Michael Hooper uncertain after he pulled out of the tour saying he was not in the right frame of mind to play, there will be concern a year out from the Rugby World Cup in France.
"Massive disappointment. Not good enough," Rennie told reporters.
"You could see there were four tries that were kicks in behind us. We certainly got dominated in the collision area.
"Rucks not a disaster, we just weren't clinical enough.
Rennie played down the number of players missing from his selection as the cause of the defeat and felt the team he put on the pitch had enough in them to get a win.
Australia return home to take on South Africa in two more Rugby Championship clashes, the first of which is in Adelaide in a fortnight.
Photo: JUAN MABROMATA