I can also drive my car 120 km/h down suburban streets with a blood alcohol level of 0.32; nobody will physically STOP me from doing it. That doesn't make it legal.
But how are they going to find out? I mean on the road it's quite easy with under cover police cars, speed cameras and the occasional chance of causing traffic chaos. But in a house? The only way the ATO can find out is if they follow you home which I doubt they'll do.
Um, it is, and they do... they tell you what the laws are, and impose huge penalties if you're found to have lied. You sign your tax return stating that all the information is correct. Surely everybody knows that fraud is a crime, don't they?
Actually penalties aren't huge. I knew someone who failed to claim tax on their 500k annual income. She got caught and was only forced to pay back half of that, in the end it worked out to be about the same as what she would have paid in taxes if she did it properly.