http://en.wikipedia.org/wiki/Island_gigantism
To be honest, I've lived in Australia my entire life and I've never seen a spider bigger than 2cm. I'm sure there's some indigenous species of spiders and insects in the remote regions that are larger, but no more than you'd expect from a country with a wider biological diversity. There's also the fact that most of these spiders live in remote and uninhabited areas of Australia, which is of course the kind of areas tourists like to visit, so of course they come home with the impression that the entirety of Australia is covered with deadly, hideous species.