Sydney has been drenched by more than 35 millimetres of rain in less than a day as roads and homes start to flood and with no reprieve in sight until next week.

"There's another 10-20 millimetres quite likely [to fall] today with several isolated places picking up a bit more than that," said Jacob Cronje, a senior meteorologist at Weatherzone, a Fairfax-owned service.

Since 9am on Tuesday the Sydney basin has had widespread rainfall reaching more than 35 millimetres at Observatory Hill and heavier in the outer suburbs.

Nearly one-third of the rain in the city's centre, or 11 millimetres, fell between 4am and 7am on Wednesday morning.