Go swimming? Watch out for the toxic box jellyfish, and there's a good chance you'll accidentally run into the little, tiny, itty bitty Irukandji jellyfish, which is nearly transparent and less than 1" in size. But they'll both kill you. Try to hug the bottom, well, you'll die there too when you get too close to a blue ringed octopus. And then your corpse will be ripped to shreds by a great white shark.
If you venture on land, you'll want to avoid the snakes... I believe that 7 of the world's 10 most venomous snakes call Australia home. So you'll want to stay in your yard... Watch out for the Bulldog and Jack Jumper Ants... They can kill you too... Want to stay in the big city? One of the world's most venomous (and most aggressive) spiders is the Sydney Funnel Web Spider... Yeah, they'll kill you too!
Mark