The Bureau of Meteorology's latest three-month climate outlook shows 70 per cent chance of La Nina - and the rain that comes with it - returning as early as September.

Off the back of two wet years, already saturated catchments are even more likely to flood again, and the forecast conditions have SES units on watch.

SES South East zone commander, superintendent Matthew Price, said crews are bracing for a repeat of recent seasons.

"What we've seen this last summer is what we're expecting to see next summer - constant wet," he said.

"It means when we do get a bigger weather event, like an east coast low, that has a greater effect on us because the catchments are already saturated.

"It's exactly what we have seen this year, and it will be more of the same, unfortunately."

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The spring and summer of 2022-23 stands to be, potentially, the third year of summer La Nina.

It's a rare occurrence, but it does happen. The last instance of a triple La Nina in this context was during the summers of 1998-2001.

Areas including Sussex Inlet, St Georges Basin, and the lower Shoalhaven lakes (i.e. Lake Conjola) are tipped to bear the brunt of flash flooding - should the La Nina occur - though most low-lying areas on the South Coast are likely to be affected in some way.

In light of the outlook, the SES is urging residents to consider joining their local unit.

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Many crews have been on the go since the Black Summer bushfires, which were immediately followed by successive floods and storms; volunteer fatigue is a concern.

"Our crews have done a fantastic job over the last two years, being there for the community," Mr Price said.

"We have an amazing group of volunteers in the South Eastern zone, but we're always looking for more people.

"In Eurobodalla we have the Moruya and Batemans Bay units, and then further north we have Ulladulla, St George's Basin and Nowra."

For information on joining the SES, visit: ses.nsw.gov.au