A La Nina has developed in the Pacific Ocean, and the south-east could be in for a wetter than normal spring and summer, the Bureau of Meteorology has declared.
The bureau has upgraded its La Nina watch from alert to active.
It says a cooler phase in the central and eastern tropical Pacific Ocean is likely to remain until at least the end of the year.
But trade winds will warm areas of the ocean north of Australia, lifting convection levels and bringing more rain to much of the country.
La Nina will also bring cooler days and more tropical cyclones.
"You need to be prepared for more tropical cyclones, earlier tropical cyclones and also wetter conditions over the coming months," the bureau's climate operations manager, Andrew Watkins, said.
The risk of flooding is increased as well in many areas.
The last La Nina was from 2010-2012 and resulted in one of Australia's wettest two-year periods on record.
Widespread flooding affected more than 200,000 people across Queensland as some towns received record rainfall. More than 30 people died.
Tropical cyclone activity in the 2010-2011 season was near normal, but five were in the severe category.
They included Tropical Cyclone Yasi, which caused widespread damage to far north Queensland.
The Bureau shifted from La Nina watch to alert August 18.
US scientists agree with the forecast, with the National Oceanic and Atmospheric Administration declaring an active La Nina status on September 10.
