The Bureau of Meteorology declared on Tuesday that La Nina has developed in the Pacific Ocean, upgrading from a La Nina 'Alert' status to an active event.

This means that recent changes in ocean temperatures and weather patterns over the Pacific are now likely to remain until at least the end of the year.

La Nina is the cool phase of the El Nino Southern Oscillation and is associated with cooler than average sea surface temperatures in the central and eastern tropical Pacific Ocean.

La Nina events often form in autumn or winter, then decay in late summer. The greatest impact normally occurs during the spring and early summer period.

La Nina events normally last for around a year, but they can be shorter, or much longer.

Recent observations and model forecasts show the central tropical Pacific Ocean is now 0.8°C cooler than normal, and that has resulted in changes to Trade Winds and pressure patterns. Climate models suggest these patterns will continue until at least the end of the year.

La Nina typically results in above-average spring rainfall for Australia, particularly across eastern, central and northern regions. It can also mean cooler days, more tropical cyclones, and an earlier onset of the first rains of the wet season across the north.

The last La Nina event occurred from 2010-2012 and resulted in one of Australia's wettest two-year periods on record. Widespread flooding occurred in many parts of Australia associated with the record rainfalls.

Tropical cyclone activity in the 2010-2011 season was near normal. However, five of the tropical cyclones during 2010-11 were in the severe category, which is above average, including Tropical Cyclone Yasi, which caused widespread damage to far north Queensland.

It is likely this year will not see the same intensity as the 2010-11 La Nina event, but is still likely to be of moderate strength.