Much of coastal NSW from the south of Sydney to the Victorian border will face the wrath of a complex low-pressure system that will bring heavy rain, hazardous surf and even flash flooding in places.

The Bureau of Meteorology has issued several warnings for severe weather, with a trough expected to deepen, generating "multiple low-pressure centres" late on Saturday and into Sunday.

Several places had already clocked up more than 150 millimetres of rain in the 24 hours to 9am on Saturday, including 154 millimetres at Nowra. That town collected another 38 millimetres by late on Saturday and the bureau is predicting 70-120 millimetres to fall through Sunday.

"Heavy rainfall, which may lead to flash flooding and riverine flooding, is forecast for the South Coast and Illawarra districts, and eastern parts of the Snowy Mountains and Southern Tablelands districts today and Sunday," the bureau said.