The Bureau of Meteorology warning, published just before 5am AEST, said heavy rainfall would develop around parts of the Illawarra, South Coast and Southern and Central Tablelands regions from early today.
Six-hourly rainfall totals between 50mm and 90mm are likely, with isolated totals of 120mm.
And 24-hourly rainfall totals are expected to fall between 100mm and 150mm, with isolated totals of 250mm.
Flash flooding is also a possibility, locals have been warned.
Conditions are expected to ease on Monday morning as the deepening trough bringing the rain moves south.
A flood watch warning was also issued yesterday evening for the Nepean and Georges River regions in Sydney, saying moderate flooding was expected today.
The rain from today’s trough is expected to add to catchments already wet with recent rain.