If both games are decided by exactly one goal, it will come down to goals scored. So for example, if Australia wins 1-0 and Denmark lose 3-2, the Danes would advance.
If that is the same, it comes down to fair play points - the team with the best disciplinary record would advance. Denmark have currently had four yellow cards to Australia's three.
If that ends up the same as well, lots will be drawn. Let's hope it doesn't come to that - imagine if, after the full time whistle has gone with France beating Denmark 2-1 and Australia leading 1-0, Mat Ryan gets a 94th minute yellow for time-wasting? Yikes.