A penalty from man-of-the-match Julian Draxler came amid debut international goals for midfielders Lars Stindl and Leon Goretzka as Joachim Low's experimental line-up dazzled at the Fisht Stadium.
But they were forced to endure a nervy finish on account of goals from Tom Rogic and Tomi Juric either side of half-time – neither of which Leno was able to reflect upon with any fondness.
Celtic star Rogic lashed a shot underneath the diving 25-yard-old after his initial attempt cannoned back off Shkodran Mustafi, while Juric survived calls for handball sent upstairs to the video assistant referee after the former's initial shot struck his arm.
Leno shelled the deflected drive to leave the Australia striker with a simple finish and possibly damaging his standing as Low continues to rotate between the Bayer Leverkusen man, Marc-Andre ter Stegen and Kevin Trapp in the injury absence of Manuel Neuer.
"Australia in the first half didn't really create many opportunities. The first goal was not really easy to stop and on the second the free-kick was deflected," Low said.
"He could have held on to that ball but he is a good goalkeeper. I have no doubt about it. In training he makes a good impression."
Speaking to reporters in the mixed zone, Leno was frank enough to take his portion of the blame.
"I think with the second goal it is obvious that I have to secure the ball, even if when the Australian player touched it he was just one or two metres ahead of me," he said. "No doubt.
"With the first goal against us it looked unlucky too, but I think it was way harder because first I moved towards the other corner.
"The shot was powerful and of course it looked unlucky but it is not as easy as it looks."
Germany are back in action against Copa America holders Chile in Kazan on Thursday.