The Bureau of Meteorology (BOM) has announced a La Niña alert, signalling a 70 per cent chance that the rainfall-encouraging climate driver will form in the coming months.
The BOM has indicated the shift from La Niña watch to La Niña alert has come after the eastern Pacific Ocean continued to cool throughout September.
Six of the seven climate models the BOM monitor now indicate Australia will reach La Niña status by November.
Andrew Watkins, head of the bureau's operational climate services, said if an event eventuated this year, it was looking like it would be weak to moderate.
"So not looking at a big event like we had back in 2010 to 2012 and not looking at anything extreme like that one," he said.
"But we are certainly looking at a high chance of having a weak-to-moderate event this year."
La Niña conditions traditionally encourage a wetter-than-average spring and summer for northern and eastern Australia.
Dr Watkins said La Niña also brought an early start to the northern wet season.
The BOM's severe weather outlook, released on Monday, warned of an average to slightly above-average risk of tropical cyclones this season and above-average risk of flooding.
"That could be a bit wet for the harvest, unfortunately, if we do get a La Niña pushing on in November," Dr Watkins said.
"But let's keep our fingers crossed for those that have got a great crop out there at the moment."
Wild weather is coming
Heavy rain has fallen over Central Australia and parts of Queensland this week thanks to an atmospheric river streaming across the continent.
Hail was reported in Alice Springs on Monday, Birdsville recorded 23.4 millimetres in the 24 hours to 9:00am on Tuesday, while 75mm fell at Mount Glorious and 35mm at Brisbane.
"In fact, when I had a look at the observations, Birdsville was the wettest place in Australia late Monday afternoon, which is pretty unusual to see," Dr Watkins said.
Over the past week, large parts of the country have recorded falls of between 10mm and 50mm, including in the south-west and for Tasmania.
"And we had that event a couple of weeks ago, so the landscape's getting a bit wet out there," Dr Watkins added.
But attention is turning to the Bight, where a front and low pressure system are developing.
"Over the next few days, we are expecting to see rainfall up the east coast again; a possibility of a further 10mm to 50mm right up through the eastern half of New South Wales and into south-east Queensland."
Widespread impacts are expected from Wednesday, but Thursday is currently looking like the big day for widespread storms in the south-east, with heavy rain, severe thunderstorms, snow and strong winds on the forecast.
The wet conditions are expected to linger on Friday before things clear up for the weekend.
© ABC 2021
01:32 AEDT Queensland's Darling Downs is reeling from severe storms that hit "like a freight train", bringing a tornado, large hail and a deluge of rain.