But the Wallabies had the edge with Aidan Toua and Luke Morahan going over in the first-half before Kimami Sitauti's brilliant try five minutes from time sealed the win.
And Mapletoft said: "It was a harsh lesson for us, they were very organised in attack and kept the ball well and we just made too many missed tackles and soak-ups.
"Against a team with the attacking prowess that they have, that is clearly going to have an impact.
"I thought at times our commitment was super, at the start of the second half we could have folded at 20-8 and a couple of early tries would have been game over but we dug in."
Australia will meet old foes New Zealand in the final after the holders thrashed South Africa 36-7 in the other semi-final.
Wales suffered a heart-breaking penalty shoot-out defeat to hosts Argentina in the fifth-place semi-final.
After the match finished in a 19-19 draw and sudden death extra time couldn't separate them the first shoot-out in tournament history was needed.
They still couldn't be separated after the first five kicks but in sudden death Pontypridd scrum-half Rhys Downes missed and Argentine winger Fernando Luna scored to secure a dramatic 9-8 win.
France will take on Argentina in the fifth-place final after they beat Fiji 44-9 - No.8 Antoine Erbani scoring two of their five tries and Gilles Bosch adding 19 points with the boot.
Scotland and Ireland will do battle in the ninth-place final. Ireland beat Samoa 37-10 thanks to two tries from Andrew Boyle while 13 points from Duncan Weir helped Scotland past Tonga 28-8.