This is the first time Australia has reached the knockout stage since 2006, and the only time they have made it past the group stage.
The Socceroos held Denmark to a draw at Al Janoub Stadium before making them counterattack in the 60th minute when Leckie broke into the Danish box, defender Joakim Maehle fouled his foot and shot past goalkeeper Kasper Schmeichel.
Denmark coach Kasper Hjulmand used all the offensive firepower he could muster as he tried to find goals that had gotten out of hand in Qatar.
But Australia has continued to take second place in Group D after France. A second-half goal helped Tunisia beat France – but Australia’s win over Denmark kept them out.