The MLHD's director of public health, Tracey Oakman, issued the alert yesterday evening ahead of two days of predicted thunderstorm activity and strong winds.

Minor to moderate flooding is expected from today, affecting the Tumut River, Murrumbidgee River and Mirrool Creek. The warning came as the Bureau of Meteorology officially declared a La Nina weather event.

Heavy rain of up to 65 to 105 millimetres is predicted from now until Friday, prompting the SES to release a flood watch warning for areas along the Murrumbidgee River including Narrandera and Darlington Point.

The public is being reminded to keep an eye on the forecast and to never drive through flood waters.

Farmers within flood watch areas are encouraged to move livestock and equipment to higher ground.

The bureau's head of operational climate services, Andrew Watkins, said much of the nation could expect a cooler and wetter summer as a result of the La Nina event.

"La Nina increases the chance of cooler than average daytime temperatures for large parts of Australia," he said.

"La Nina also occurred during spring and summer of 2020-21 ... back-to-back La Nina events are not unusual."

Dr Watkins said this year's event was not likely to be as strong as the 2010-12 event, but rain is likely to persist until January.