Sydney is in for a miserable afternoon as drenching rain sets in and winds intensify to reach potentially damaging speeds along the coast later in the day.

The falls bring a sodden end to what is likely to be the city's wettest March in 42 years. The latest source of rain is the moisture being drawn in by the remnants of powerful ex-tropical Cyclone Debbie, which has combined with a cold front that has swept in from the south.

The Bureau of Meteorology has updated its severe weather warning covering coastal NSW from Sydney to the Queensland border. A separate warning is out for severe thunderstorms bringing potentially heavy rain to the lower Hunter including Newcastle and Gosford.

"24-hour totals in excess of 200 millimetres are expected over the Northern Rivers district during Thursday, and it is likely that some locations will exceed more than 350 millimetres," the bureau said.