In general terms, we have the same problem as other "small" countries, in that we have a limited pool of developing players. Australia simply has five times more. And that is enough to make a difference where you would have something to work with, and the environment is competitive (the best go on to represent the country).
Both Australia and New Zealand suffer from football not being their top sport, lots of kids grow up playing something else (proportionally more than on the Continent). But even among the European and South American countries, where football gets the best young athletes, those countries that have large populations tend to do well in top tournaments (Germany, UK, France, Brazil, Italy, Argentina, Mexico, Spain). Russia and Ukraine are an exception.
There are "smaller" country examples that are positive (successful) exceptions like the Netherlands, Greece, Croatia, Belgium, Portugal or Denmark, but even their teams go through performance swings while the large country teams tend to be more consistent and do well regularly.
Both Australia and New Zealand suffer from football not being their top sport, lots of kids grow up playing something else (proportionally more than on the Continent). But even among the European and South American countries, where football gets the best young athletes, those countries that have large populations tend to do well in top tournaments (Germany, UK, France, Brazil, Italy, Argentina, Mexico, Spain). Russia and Ukraine are an exception.
There are "smaller" country examples that are positive (successful) exceptions like the Netherlands, Greece, Croatia, Belgium, Portugal or Denmark, but even their teams go through performance swings while the large country teams tend to be more consistent and do well regularly.