Nowra has received almost 200mm of rain in just 24 hours, according to the Bureau of Meteorology.
Between 9am on Sunday, February 9 and 8am Monday, February 10, the region recorded 193.2mm of rain.
This was on the back of more than 100mm since Thursday, February 6. The Shoalhaven River and creeks are rising, and the SES has received a number of calls fro help, Nowra unit media liason officer Jacqui Gilmore said.
"We have had quite a few calls for leaking roofs, and there has been a report of a tree on a house," she said.
"We are worried about moderate, localised flooding, particularly at Shoalhaven Heads and Greenwell Point. There are already houses, garages and outbuildings with water in them, and the river has gone from 2.8m to 3m."
She reminded homeowners who may have plugged their gutters to protect against fire to remember to unplug them, and encouraged people to listen to ABC radio for any emergency updates.
Sandbags have been dropped to Shoalhaven Heads and Greenwell Point RFS stations for residents to use as necessary.