Australia has 20 million people, give or take, in a country slightly less mass than the US.
After dark, there are probably 100,000-200,000 people outside, driving or whatever. 75% of those would be on roads, not looking at the sky. That leaves only a few thousand people available to look at the sky. lets call it 50,000 for arguements sake.
So 50,000 people available to look at the sky covering a land mass 7,619,730 sq km = 152 sq kilometres per person to look at. An impossible task. After midnight, there would be 10,000 people max out and about, more like 5,000. Therefore that explains the lack of sightings in Australia.
US land mass 9,629,091sq km, population 300 million = .032 sq km per person. Simple maths explains the huge number of sightings in the US.
Of course this is very basic calculations, but gives you an idea of how things work.