The third La Nina event in a row, combined with a continuing negative Indian Ocean Dipole will likely mean Australia's east coast is set for holiday season downpours.

Bureau of Meteorology's climate driver update revealed tropical Pacific sea surface temperatures have been cooling since June and are now at La Nina thresholds.

The La Nina event may peak during the spring and return to normal conditions in early 2023, according to BOM.

Rivers and streams are expected to flow strongly this spring.

Between September and November, high-stream flows are predicted around Australia, after flows were high at 67 per cent of locations tested last month, BOM revealed in its Seasonal Streamflow Forecasts.

A negative Indian Ocean Dipole event is still under way, which increases the chance of rain, and is likely to continue into late spring.

Meanwhile, NSW State Emergency Service (NSW SES) Commissioner Carlene York said while the agency was well-prepared for another busy period, it was critically important for people to take responsibility and be prepared.

"Today's declaration means eastern Australia is heading into a third consecutive La Nina season, which presents unique risks and challenges," Commissioner York said.

"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.

"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."

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