How do I turn in an illegal Australian?

He is laughing at me and quite smug about being here for the past 4 or 5 years without having to pay taxes, living off a "cougar" and partying on. I understand Mexican immigration based on the geographic location, but Australia? This country is on the other side of the planet and the person had to have either FLOWN in or taken a SHIP. Somewhere along the line the USA has dropped the ball, how do I turn him in?