Severe weather warnings for rain, winds and flooding have been issued for coastal areas of the eastern state.
Australia's Bureau of Meteorology (BOM) warned of "dangerous conditions" on Saturday and Sunday.
There has already been flooding in Sydney and other areas along the coast.
Friday was the wettest day recorded in well over a year in Sydney, where roads were closed and public transport delayed.
Other NSW towns faced flood waters as well, including Byron Bay and Coffs Harbour, where 280mm and 250mm of rain fell respectively.
The heavy rains are expected to continue until early next week, providing relief to some drought and fire-ravaged areas.
NSW Rural Fire Service said the rain had extinguished a third of the blazes there, but as of Friday, 43 were still burning.
"Good rainfall is being recorded in parts of the state, with a hope it continues to drop where needed most," the fire service said.