Repeat hydrographic sections occupied in 1995 and 2005 reveal a rapid decline in the salinity and density of Antarctic Bottom Water throughout the Australian Antarctic Basin. The basin-wide shift of the deep potential temperature-salinity ($theta$ - S) relationship reflects freshening of both the Indian and Pacific sources of Antarctic Bottom Water. The $theta$ - S curves diverge for waters cooler than -0.10C, corresponding to a layer up to 1000 m thick over the Antarctic continental slope and rise. Changes over the last decade are in the same direction but more rapid than those observed between the late 1960s and the 1990s. When combined with recent observations of similar freshening of North Atlantic Deep Water, these results demonstrate that dense water formed in both hemispheres is freshening in response to changes in the high latitude freshwater balance and rapidly transmitting the signature of changes in surface climate into the deep ocean.