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

During La Nia, 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.

The Bureau's Head of Operational Climate Services, Dr Andrew Watkins, said that typically during La Nia events, rainfall becomes focused in the western tropical Pacific, leading to wetter than normal period for eastern, northern and central parts of Australia.

"La Nia also 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.

"La Nia is also associated with earlier first rains of the northern wet season, as we've observed across much of tropical Australia this year.

"The last significant La Nia was 2010-12. This strong event saw large impacts across Australia, including Australia's wettest two-year periods on record, and widespread flooding.

"La Nia also occurred during spring and summer of 2020-21. Back-to-back La Nia events are not unusual, with around half of all past events returning for a second year."

Dr Watkins said that this year's event is not predicted to be as strong as the 2010-12 event and may even be weaker than in 2020-21 La Nia event.

"Every La Nia has different impacts, as it is not the only climate driver to affect Australia at any one time. That's why it is important not to look at it in isolation and use the Bureau's climate outlooks tools online to get a sense about likely conditions for the months ahead," Dr Watkins said.

The Bureau previously shifted to La Nia watch on September 14 2021, and to La Nia alert on October 12 2021.

La Nia is likely to persist until at least the end of January 2022.