Well, here's the thing. There's no proof that it is impossible for them to do. You don't know their manpower or hours.
Extrapolation from known data: there are currently 7,108,000,000 people in the world. The United States has a population of 316,608,222.
Even if every single member of the United States was an NSA agent and the NSA was attempting to monitor the entire world, each person would be monitoring 22.45 people simultaneously.
Let's make things a bit more realistic. According to the NSA, there are
between 30,000 and 40,000 employees. Assuming every single one of those 40,000 was an agent monitoring worldwide communications (which is obviously false, as this includes everyone from the mail room to the top dog in the whole organization), each agent would need to monitor all of the communications of 177,700 people. Individually.
The chance of some random Australian schmuck being monitored for no reason other than "worldwide surveillance" is very, very slim.