Just in case you don't follow the goings-on in the country/continent, South Australia is in the middle of a serious drought. Things seem to keep getting worse, and if it keeps up something is going to have to give. The good news is that it is unlikely that grape vines would be the first to be asked to make a sacrifice. For starters, it's good for tourism. More importantly, grapes need less water than many other crops. This article gives a description about what exactly is going on.
The bad news is not that the quality of Australian wine will diminish. That's not how winemaking works. What's happening is that winemakers are having to dispose of more grapes because they don't make the cut. They can still select the very best grapes to make their wines, there's just fewer of them. That could mean higher prices. I doubt very much we would see a dramatic rise in price - the Australian government would probably subsidize the winemakers if things got too bad. Hopefully the rains will come soon and we can continue to enjoy that rich, fruity Shiraz.