The the department of the-Australian-Bureau-of-Meterology:
With the exception of the Southern Oscillation Index (SOI), all ENSO indicators showed an intensifying La Niña during September. Computer models forecast the La Niña to last until early 2008, making it almost certain that 2007 will be considered a La Niña year.Finally, finally, finally! Wet weather is now due for Australia's South-East.
Most critically, the near-equatorial Pacific Ocean has continued to cool both on and below the surface, the Trade Winds remain stronger than normal across the western to central Pacific, and cloudiness in the equatorial Pacific is reduced. Together, these indicators suggest the atmosphere and ocean are reinforcing each other; a critical component in sustaining La Niña conditions for any period of time.
However, this La Niña has been late to develop by historical standards. In the past, most significant La Niña events were established by winter's end, with widespread above-average rain falling over Australia's eastern half. With a late-developing La Niña, this typical rainfall response is not as likely as in past episodes. Indeed it has been largely absent to date.
Moreover, Australia's climate may continue, at least in the short term, to be influenced by the unusual state of the oceans to the north, and particularly northwest, of the continent. These have been cooling since June when, historically, they would have been expected to warm as the La Niña evolved in the Pacific. These cooler than normal waters inhibit the formation of northwest cloudbands, which are a major source of winter and spring rain for central and southeastern Australia during La Niña years.