In a blow to communities recovering from record flooding, a La Nina weather system will bring further heavy rainfall for a third consecutive summer.
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With dams already full and soil inundated across much of northern and eastern Australia, people have been reminded it is critical to prepare for a weather emergency. Some people who've never experienced floods may also be at risk.
On Tuesday, the Bureau of Meteorology officially declared a La Nina weather system is underway in the South Pacific. The weather system is set to dampen holidaymakers' plans with downpours expected over the festive season.
Models predict the event, characterised by above average rainfall, will continue until early 2023. It is expected to peak during spring.
Many communities are still rebuilding after at least 22 people died and more than 25,000 homes and businesses were damaged by floodwaters earlier this year. In the first week of April southern Queensland and northern NSW each received more than a year's worth of rainfall.
This is the third back-to-back La Nina event and the fourth instance of three consecutive events since records began in 1900.
NSW SES Commissioner Carlene York said while the agency was well-prepared for another busy period, it would be critically important for people to take responsibility and be prepared.
She warned another La Nina weather season would present unique risks and challenges.
"There is already wet soil, high rivers and full dams right across our state, and with more rain on the horizon comes the very real possibility of flooding," Ms York said.
"If you live in a flood-prone area, I urge you to take steps to prepare now. Make sure you know your risk, have an up-to-date emergency plan and emergency kit.
"Preparing early will save you vital time during an emergency."
According to the Bureau's Climate Driver Update, released on Tuesday, tropical Pacific sea surface temperatures have been cooling since June and are now at La Nina thresholds. Atmospheric indicators are also displaying patterns typical of the weather event.
Bureau head of long-range forecasts, Dr Andrew Watkins, said that while La Nina criteria had been met, most models forecast it to be weak to moderate in strength.
"During La Nina events, waters in the eastern tropical Pacific Ocean are cooler than normal, and waters in the western tropical Pacific Ocean warmer than normal. This causes changes in wind, cloud and pressure patterns over the Pacific," he said.
"When this change in the atmosphere combines with changes in ocean temperature, it can influence global weather patterns and climate, including increasing rainfall over large parts of Australia."
Dr Watkins claimed there was an 80 per cent chance of above average rainfall for many parts of the eastern half of Australia.
Environmental advocacy group Climate Council research director Dr Simon Bradshaw said the La Nina would bring tough times ahead for many Australians.
"The risk of extreme rainfall and flooding is also exacerbated by climate change, driven by the burning of coal, oil and gas," Dr Bradshaw said.
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"Helping vulnerable communities build their resilience to, and ability to recover from, worsening flood disasters must be a top priority for state and federal governments, and will help minimise the dangers and devastation of yet another La Nina event."
According to the Bureau, a negative Indian Ocean Dipole (IOD) event also continues. A negative IOD is associated with above-average spring rainfall for much of Australia. When a La Nina and negative IOD combine, it further increases the likelihood of above average rainfall over Australia, particularly in the eastern half of the continent.
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