The Big Mac is the most iconic of McDonald's burgers (and quite easy to make at home). I've seen a few posts popping up on Facebook recently suggesting that McDonald's is about to dump the Big Mac -- a classic example of people passing on a rumour without checking the source.
Myth-busting site Snopes has a neat deconstruction of the rumour, which appears to have originated with an entirely fake news article on satirical site Daily Buzz Live. While it's true that McDonald's in the US is looking at simplifying its menu as a means of combatting sales declines and speeding up service, that doesn't involve eliminating the Big Mac.
Even if it did, it wouldn't automatically follow that this would happen in Australia. McDonald's down under operates a quite distinct menu, though that doesn't stop it being the subject of rumours that don't apply here (such as the infamous "pink slime" problem).
Not Lovin' It [Snopes]