An overnight drenching has dumped more than 50 millimetres of rain on Sydney's CBD and surrounding suburbs, delivering the Harbour City's wettest day since February.

And the wet and windy conditions are expected to persist throughout Wednesday, with the city's east expected to bear the brunt of a low-pressure system off the NSW coast.

Isolated thunderstorms could hit the city's east during Wednesday morning, while the Bureau of Meteorology is forecasting about another 20 millimetres of rain to fall through the day.

The "feels like" temperature dipped to an icy 2.9 degrees in the city at midnight as winter made its presence felt, before heavy rain started hammering down a short time later. The heaviest falls set in at 12.30am in the city, and continued throughout the early hours of the morning.