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"
by KinGAleX April 04, 2005