Australia is officially in a La Nina phase, albeit a weak one, according to the Bureau of Meteorology.

The bureau does not expect to see the above-average rainfall that normally accompanies a La Nina event, but there is an increased chance of prolonged warm spells for south-east Australia.

The El Nino-Southern Oscillation outlook was raised to a La Nina level last Wednesday and climate models suggest it will persist until early autumn.

Signs of La Nina increased through spring, with the central and eastern tropical Pacific Ocean cooling steadily since winter. Ocean temperatures are now at La Nina thresholds of 0.8 degrees below average.

Atmospheric indicators, including the Southern Oscillation Index, trade winds and cloud patterns, also signal a La Nina.

The event must last three months for 2017/18 to be classified a La Nina year.