WATCH

La Nina brings more rain for central and eastern Australia.

The Bureau of Meteorology has warned that more rain and flooding is on the way for central and eastern Australia after officially declaring a La Nina weather event on Tuesday.

BoM meteorologist Jonathan How said NSW, Victoria, Queensland and the ACT could expect rain in the coming days in a severe weather update on Tuesday.

"Accumulated rain totals show moderate to large falls building across the eastern two thirds of Australia for the rest of the week, with Tasmania the exception," Mr How said.

"Storms will bring high isolated totals especially on Thursday and Friday. A series of fronts and troughs will trigger activity initially but into Wednesday, a low pressure system will deepen over the southeast. This low tracks towards southern NSW, the ACT and eastern Victoria, which will become the focus of wind and rain.

"Rain and storms will also extend up the east coast, easing by the weekend. And it will be cool to cold behind the system. Flood impacts will however continue for days if not weeks to come."

NSW SES flood rescue operators launch a flood boat off the banks of the Lachlan River on November 17, 2021. Major flood warnings are still in place for parts of the Lachlan River.

NSW SES flood rescue operators launch a flood boat off the banks of the Lachlan River on November 17, 2021. Major flood warnings are still in place for parts of the Lachlan River.

Mr How said the rain and storms would "bring the potential for further flooding and significant impacts on communities".

"Multiple flood warnings are still in place, including major flooding for parts of the Lachlan, Namoi and Gwydir rivers," the meteorologist said.

"Upcoming rain may worsen current flooding. An initial flood watch has also been issued for parts of South Australia where heavy rain may cause overland flow and disruptions to transport."

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Mr How advised people continuously check flood warnings even if they were outside of a flood area as conditions could rapidly change.

The meteorologist said the wild weather was being caused by two systems combining to drive "humid and unsettled weather".

A map of flood warnings for central and eastern Australia. There are major flood warnings for the Lachlan, Namoi and Gwydir rivers and an initial flood watch for parts of South Australia.

A map of flood warnings for central and eastern Australia. There are major flood warnings for the Lachlan, Namoi and Gwydir rivers and an initial flood watch for parts of South Australia.

"To our northwest, tropical cyclone Patty is pumping cloud and humidity into the centre of the continent. At the same time, a humid air mass has been sitting over the east, bringing large totals in the past 24 hours," Mr How said.

"These two tropical air masses will combine to bring widespread falls.

"It's been a wet spring for large parts of the country and even though summer is approaching, rain and flooding isn't over just yet."

BoM officially declared a La Nina weather event on Tuesday, saying the Pacific Ocean was in a La Nina phase.

Historically, La Nina events correlated with wetter than average conditions in many parts of Australia, especially eastern, northern and central regions.

La Nina is part of a cycle known as the El Nino-Southern Oscillation (ENSO), a naturally occurring shift in ocean temperatures and weather patterns along the equator in the Pacific Ocean.

BOM head of operational climate services Andrew Watkins said that during La Nina waters in the central or eastern tropical Pacific become cooler than normal, persistent south-east to north-westerly winds strengthen in the tropical and equatorial Pacific, and clouds shift to the west, closer to Australia, leading to increased rain over the continent.

Dr Watkins said the La Nina event was correlated with the likelihood of a wetter than average wet season in areas of the country with summer dominant rainfall.

He said much of the nation could expect a cooler and wetter summer as a result of the event.

"La Nina increases the chance of cooler than average daytime temperatures for large parts of Australia and can increase the number of tropical cyclones that form," Dr Watkins said.