Much of the continent is already soaked by spring rain.
The Bureau of Meteorology has added to flooding fears with the La Nina being reinforced by other strengthening climate drivers.
Combined, they have increased the likelihood of above average rain for northern and eastern Australia right into the summer.
The most serious flooding concerns are still centred on New South Wales but after this week's drenching there are flood warnings in Queensland, South Australia and Victoria.
The mammoth Dartmouth Dam at the headwaters of the Murray River is spilling for the first time in 26 years, good news for irrigators in the years to come but heightening the flood risk downstream.
The constant rain is threatening crops from west to east.
The bureau this week said the La Nina, the nation's record-breaking third in a row, is now well established and still expected to peak in spring and return to neutral early next year.
University of Melbourne senior lecturer in climate science Andrew King said most seasonal prediction models are suggesting this La Nina event will be weaker and shorter-lived than the last two.
"While we've seen triple La Nina events before, we have never seen quadruple La Nina in the historical record. This doesn't mean it couldn't happen," he said.
But there has been movement with the other climate drivers in Australia weather.
Sea surface temperatures in the tropical Pacific have weakened "a little", the Southern Oscillation Index has continued to rise and is currently well above La Nina thresholds.
The negative Indian Ocean Dipole event continues as well, the bureau's experts say.
Computer models indicate the negative IOD is likely to persist at least until late spring.
"A negative IOD typically increases the chance of above average spring rainfall for most of the eastern two thirds of Australia," BOM said.
"When La Nina and negative IOD conditions combine, the likelihood of above average rainfall over Australia is further increased, particularly for the eastern half of the continent."
The Southern Annular Mode is currently positive and the bureau says it is likely to remain generally positive throughout spring into early summer.
During spring, positive SAM increases the chance of above average rainfall for parts of eastern NSW, eastern Victoria, and south-eastern Queensland.
The Madden-Julian Oscillation is strengthening and will "maintain moderate strength as it moves east towards the western Pacific".
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The MJO increases the chance of above average rainfall over the eastern half of Australia.
Another sign of the returning La Nina, forecasters say, are the storms forecast for Darwin later this week which could signal an early triggering of the northern rainfall onset.
The onset, the official arrival of the wet season, typically occurs in parts of coastal Queensland and the Top End from late October or early November.
Don White of Weatherwatch says there is no indication of significant change in the current wet weather pattern in the foreseeable future.
"In other words, above average rainfall is expected to continue for the rest of the year at least, although there may be a bit of a trend to see the largest rainfall anomalies move slightly to the north," Mr White said.
He is forecasting the La Nina may not return to neutral until early autumn 2023.
Mr White said La Nina years are also associated with a slightly above average number of tropical cyclones in the north from December to April .
Early cyclones can "be a source of upper level moisture streaming across the continent", he said.