As a randomly selected representative of Oz (ie. the first one here today) I can safely say that I have *no idea! Had never heard of the Beyond the Black Stump site before.
Inselpeter: try this phonetically: oz-tralia. I know nothing of the site, but certainly down here the mythical "black stump" is a place that is always far, far away. It is used to illustrate the magnitude of any item in question, eg: "It's the best deal you'll get this side of the black stump" or "She has the best legs this side of the black stump" etc etc etc.