It's not actually just an Australian term for a hurricane. It's a special type which occurs in the outback, when a swirl of wind picks up wind or sand and spins it around tornado-like. This is very small scale and lasts from about 5 seconds to about 10 minutes.
"Oh look, it's another willy-willy startin' up behind the shed"
If something was completely unbelieveable/inconceivable, sometiems someone will exclaim "Sif!", a word used in internet circles. This word could be used when describing the situation of when the exclamation occured.
"The Sifness was so mindblowing, I was like, wow, that frood must be leetzor!"
The most widely sold "surf brand" in Australia, which is a complete rip off and has stolen the name of a small river in central Australia. Unfortunately considered cool until teenagers realise what "Gucci" and "Armani" are and bleed their parents dry, while talking on their mobile phones and eating fast food.
"Once a jolly swagman, camped by a Billabong, under the shade of a Quiksilver tree"