Australian meteorologists have declared a La Nina weather event is now under way, with the country's wettest spring in 10 years to continue into summer.
La Nina is part of a cycle known as the El Nino-southern oscillation, involving a natural shift in ocean temperatures and weather patterns in the Pacific Ocean, bringing high levels of rain, floods and cyclones.
Bureau of Meteorology head of operational climate services Andrew Watkins says the La Nina is expected to persist in eastern, northern and central parts of Australia until at least the end of January 2022.
He says it will bring more rain to river catchments that are already at their capacity.
"We have seen a relatively wet spring, it could be our coolest spring since 1999 and it is looking like the wettest spring since 2011," Dr Watkins told reporters in Melbourne on Tuesday.
"Because of the conditions we've seen over the last couple of months, making the landscape very wet, we are at risk of more widespread flooding than usual."
The areas most likely to be hit by heavy rainfall and flooding during the next three months include Queensland, NSW, Victoria and Tasmania.
La Nina also brings a 65 per cent higher chance that parts of the country will see more tropical cyclones than average.
While cooler temperatures will be felt in parts of eastern and southern Australia, there could also be heatwaves.
"The heatwaves we get over summer tend to be longer, although not as extreme, and more humid," Dr Watkins said.
The last significant La Nina hit Australia in 2010 to 2012, leading to the nation's wettest two years on record, with widespread flooding.
However, Dr Watkins said this year's event was not predicted to be as strong.
"Last year we saw a weak to moderate La Nina event. Now we're backing that up with a weaker La Nina event," he said.
"A weak La Nina can still bring heavy rainfall at times. With a wet landscape we are at risk of more widespread flooding over the summer."
Bureau senior meteorologist Dean Narramore said parts of Victoria, Queensland and NSW would be lashed with heavy rain and thunderstorms from Wednesday.
During the next four days, he said eastern Queensland, eastern and central NSW, and Victoria's north and east could receive between 50 and 100 millimetres of rain.
Rain and flooding is predicted to be at its worst between Thursday and Friday.
"These rainfall numbers, on already saturated soils, is likely to lead to renewed river level rises on many of our already-flooded rivers," he said.
"If you are out and about in the coming days, particularly the northern and eastern parts of Victoria, inland NSW and southern inland Queensland, just be weather aware, check the radar, check the warnings."
The bureau had initially announced a La Nina alert in October, saying it was expected to be confirmed by November.