Lake Eyre South is the lowest point in Aus, so there's nowhere else for the water to go.
Mike
BTW, I was out there last year and Lake Eyre South had some water in it, but from different feeders. The main lake was far from "full", despite all the media hype. The Lake Eyre regatta had to be held on a smaller lake, 'cos the water line was too far out for access to deep enough water for sailing.