AA, you do not know if any Australians are infected. What we know is that no suspected cases have been confirmed. What we also know is that we cannot keep the flu out, the best we can do is DELAY the onset of a pandemic if we are lucky and work hard. Thats the phase of the National Pandemic Plan we are in right now - DELAY. Thats why we have introduced thermal scanning and health cards etc., it may help us catch a case or two, but sooner or later an asymptomatic individual is going to fly in and then come down with Flu Four days later.
The other thing you need to understand is that Influenza pandemics follow an exponential law. It's a bit like a bushfire. It starts with a tiny little spark, for the first few minutes it's easy to put out, but then it gathers strength. All the stuff about washing your hands, using a handkerchief and isolating your self if you get sick, closing schools and public events is aimed at removing both fuel and sources of ignition from the Influenza bushfire.
The other thing you need to understand is that even if a Pandemic is mild, at the height of it, there could be upwards of 2 million Australians sick with it at once, even if its mild. That is going to swamp the health care system even if people don't die.